File Syncronization with Office 365 FAQ

"We couldn't sync this library." in SharePoint Online is a typical error message to receive if you try to sync document libraries to a local drive using the OneDrive for Business client. This could have many reasons. See below about how to solve all of it using the Layer2 Cloud Connector.

How to sync a local file server with Office 365 / SharePoint Online / OD4B?

Office 365 File Sync Issues Solved By Layer2 Cloud Connector 600 


When you are moving to the Microsoft Office 365 and SharePoint Online cloud, there are a lot of reasons to keep your local file system, for example:
  • To quickly access files with non cloud-based applications or legacy systems
  • To work offline - with increased performance and without internet connectivity
  • For integration into local business processes
  • For compliance reasons
  • As some kind of local backup (or to manage "cloud" backup with common backup software)

But you don't have to miss the benefits of the cloud, like web based search, versioning, change notifications, workflows, mobile access, etc. Setup a hybrid scenario:

You can synchronize selected parts of your local file system or server share with a SharePoint, Office 365 or OneDrive for Business document library in the cloud.

This article shows how this works step-by-step with the Layer2 Cloud Connector. The product can close some gaps and solve many issues that still exists with the Microsoft out-of-the-box offerings.


Office 365: We couldn't sync this library error 

Fig. If you try to sync SharePoint Online document libraries using the OneDrive for Business client you will often receive an error message like "We couldn't sync this library.". This can have many reasons. See below about how to solve all of it.


Microsoft SharePoint, Office 365 and OneDrive for Business file synchronization
Sync with Layer2 Cloud Connector 
Sync with Microsoft OneDrive
for Business Sync Client
​SharePoint 2007 / WSS 3.0 No​ No
SharePoint 2010 and Foundation Yes No
​SharePoint 2013 and Foundation ​Yes ​Yes
SharePoint Online / Office 365 ​Yes Yes​
​OneDrive for Business ​Yes Yes​
​Suport for all Windows Operating Systems ​Yes No​
​Central Administration, Logging, Alerting in case of issues ​Yes ​No
​Server-to-Server Sync ​Yes ​No
​Can sync from File Server ​Yes No​
​Can sync User Home Drives Yes​ No​
Works for any Office 365 Plan ​Yes ​No
​Unlimited amount of files ​Yes* ​No
​Any source drive supported ​Yes No​
​Replication conflict solution strategies (e.g. one entity wins, keep both etc.) ​Yes No​
​On demand sync ​Yes No​
​Scheduled sync ​Yes No​
​Uni-directional sync Yes​ ​No
​Bi-directional sync ​Yes Yes​
​No-delete option ​Yes No​
​No-insert option Yes​ ​No
​No-update option ​Yes ​No
NEW: Flexible SQL-like query language to filter files by name or path (including like and wildcards), file type, size, date etc. for migration, backup and sync ​Yes (requires 6.2 and higher) No​
NEW: Dynamic Columns feature to add specific business logic via C# e.g. to translate a file path or user name based on source properties.
​Yes (requires 7.0 and higher) No​
​NEW: Assign SharePoint managed metadata on-the-fly during migration based on file name, title, path etc. Yes (requires 7.0 and higher)​ ​No
List view support on SharePoint ​Yes ​No
​Flexible column mapping to keep metadata ​Yes No​
​Sync file metadata only to lists(without the file content), link to files located anywhere, e.g. FTP, Azure, DropBox... Yes​ ​No
​Sync to specific target folders in a SharePoint library ​Yes No​
​Support for Office 365 Groups Yes
​Support for Microsoft Flow ​Yes (via SharePoint) No​
​Support for Azure Logic Apps ​will be released soon ​No
​File name convention issues solved ​Yes ​No
​File path length issues solved ​Must be fixed manually
based on error logs
​Forbidden file types auto-zipped Yes​ ​No
​Can sync not only files but also structured data from 100+ data sources like SQL/ERP/CRM etc. ​Yes** ​No
​Licensing per installation, several editions, from Personal to Professional Free​
​Data volume-related costs  ​No No​
​User CALs required ​No No​


* Depending on configuration and amount of changes - but no hard 5000 / 25000 items limit as out-of-the-box. Please edit your SharePoint views to still be able to navigate your files: Remove all sorting, filtering, grouping. Set to flat view in the folder section in case you have folders with more than 5000 files. You can make use of SharePoint search or managed metadata to find your files without any restrictions. The "folder" parameter in connection string is not supported above the list view threshold (as it is implemented as a filter by SharePoint).

** Take a look here for supported systems and applications.

Now let's setup a sample connection to sync a selected file share directory one-way to SharePoint Online.

1. Select parts of your local file system to sync with Office 365

Let's assume you have a local file system like this, such as some folders and files in the C: drive:



Fig. Local File System to synchronize with the Microsoft Office 365 cloud - also works for server shares.

You first have to setup the connection definition in the Cloud Connector. The example "FileSystem to SharePoint Online" is shown below:


Fig. Example uni-directional connection definition for Filesystem to SharePoint Online.
Please note:
  • There is a bi-directional option available to sync back changes done online in the SharePoint library back to the local file server.
  • There is an Overwrite Destination option to use for cleanup on the initial sync, if your library already contains some data.
  • Do not enable scheduling at this point. Please enable scheduling later on, when all tests are passed and you confirm the sync works.

The next steps will be to configure the data entities for your file system and SharePoint Online/Office 365.

2. Define your local file system as data source for Office 365 replication


Fig. Example file system data source configured for replication to SharePoint Online/Office 365.

Please note:

  • Select the Layer2 FileSystem Provider as Data Provider.
  • Enter the source directory (root) with the notation Directory=C:\myRootDirectory or similar as the connection string.  In the example above, this is "C:\TestFolder\SimpleTest".
  • Network drive locations are supported. You can use UNC notation (\\myserver\myshare) or the IP address (\\192.168.X.X\myshare\). While you can use mapped drive letters, we advise against this as it often has errors.
  • Please make sure that the current account (for Connection Manager) and Backend service account have appropriate rights to access the shared drive. The Layer2 Cloud Connector service by default uses a local system account and may not have access to shares. You have to modify the accounts and / or access rights for this.
  • There are additional options available in connection string for non-recursive replication or to filter specific file types:
  • Directory: Root path where the files will be read from (mandatory).
  • UserId / Password: Optional credentials to connect to target path. Aliases are User, UserId, User Id and Password, pass, pwd.
  • FilenameFilter: Optional search pattern for filtering files by name or extension. Parameter accept standard windows file search pattern like *, ?. Aliases are FilenameFilter, FileNameFilter, filenamefilter. (Note that you can only set one filter pattern for this parameter.)


    *jpg = file extensions
    Layer2* = Files which begins with Layer2
    *Layer2* = Files that contain Layer2
  • SQL-like query is supported to include or exclude folders and files to / from sync by name (including like or wildcards), type, size and date.

3. Define a SharePoint library in Office 365 as data destination for your file system

Next, after creating a new library in SharePoint (called "Documents" in this example), you can configure the data destination.


Fig. Example SharePoint Online/Office 365 data source to replicate a part of a file system to.

Please note:

  • You have to select the Layer2 SharePoint CSOM Provider to communicate with the O365 cloud using web services.
  • Setup the connection string for example as:
    Url=; List=myList; Authentication=Office365; [email protected];
  • Password goes in the Password field, or you can add it to the connection string (it will be moved to the Password field on next UI load).
  • URL: Link to the site that hosts the library.
  • List: The name of the library you will sync to.
  • View: The name of the view you wish to use (optional).
  • Authentication: Typically set to Office365 when connecting to SharePoint Online. There are other authentication methods available, if required. See the User Documentation under the "Authentication Methods" section for this.
  • User: The email address of a user that has read/write access to the library.

4. Setup field mapping for file system metadata

To map the file system content and metadata to your SharePoint library, you can simply use the Enable Auto Mapping feature or map the fields manually - as shown below.


Fig. Example field mapping to sync file system metadata to a SharePoint Online library.

Please note:

  • You can generally map file system metadata as Created / Modified to the corresponding SharePoint fields. But note: Writing to SharePoint system fields (Modified requires specific access rights and can cause replication issues. We strongly advise to map such fields to custom columns, like FSCreated, FSModified, etc.
    The custom fields must be created before mapping.
  • Please be careful when mapping author/editor data to the SharePoint metadata fields. In some cases, the user tokens are not the same between file system and the cloud accounts, and may lead to warning about missing users in SharePoint.
  • If you can't find some list columns in mapping, they are not exposed in the view you are using for sync. If you don't give any view in connection string, no view is used and it pulls ALL columns for that library. Better to use specific view for sync. Please add the columns needed there. Remove ANY sorting, grouping, filtering to overcome the list view threshold.
  • Office document metadata is propagated by SharePoint automatically in case the columns exist with the same column name. For example, an Author column is populated with the content of an Author metadata field automatically by SharePoint. You don't have to assign such kind of metadata in the field mapping section.
  • If using bi-directional syncing, please note the conflict resolution mode (in the Mappings section). Please select depending on your requirements. For more information about the different modes, see the User Documentation under "Conflict Resolution".
  • You can also sync your file system with SharePoint lists to only add metadata and backlinks - not the files itself.

5. Access your local files via Office 365 from everywhere

​When all connection settings are completed and verified successfully, go back to the configuration settings and select Run Now to start your initial sync.




Fig. Example successful completed sync to SharePoint Online.

After all files are transferred, you can reap the benefits of your local data now being replicated and available in the cloud!


Fig. Example of SharePoint Online library with file system content replicated into it.

Please note:

  • In case of errors while replicating first time, please see the log section. Increase log level for more details, if required. (See "Logging Basics" in the User Documentation for how to configure this.)
  • Initial replication can take some time, depending on amount of data and bandwidth. Expect 2-4 seconds communication overhead per file plus the upload time. Note that Office 365 limits the upload and download speed.
  • Later on, only changed items are replicated. This is much faster compared to the initial replication. Expect around 20-40 seconds to compare metadata per 5.000 files + upload / download time for changed files.
  • To setup automatic background replication, such as every 24 hours or 20 minutes, go to connection settings. It is advised to set sync intervals based on your typical sync duration - average time to complete should be your interval.
  • Generally, there should be only low CPU and memory usage during sync. However, syncs of larger sizes will take more memory over time. See Best Practices: How to sync large amounts of local documents with Office 365 for guidance and performance metrics.
  • SharePoint has different file name, type, path, or size restrictions compared to the file system. We recommended you "clean" your file system source for restricted filenames, folders, and other items not allowed in SharePoint Online before the first sync. For more information, see How to fix sync issues between local file shares and OneDrive for Business, which includes a link to Microsoft's free "Easy Fix" tool that will check your file system for these issues.

    Otherwise the Cloud Connector will manage most naming convention issues by itself, and you can address those it cannot manually by checking the error log for the problematic files/folders.
  • Please note the SharePoint list view threshold can appear when a list or library has above 5000 items in it. This is not an item limit for the sync but a view limitation! The Cloud Connector can get around this if you use an unsorted, unfiltered, ungrouped view for replication in connection string. This view can be executed in all cases also above 5.000 items. Please index your columns to execute more complex views. Display your library as flat file list, if required, by removing the folder option in view settings. Use managed metadata navigation, folder navigation, and search within large libraries and lists without any restrictions. For best results, keep the number of files per folder below 5.000.
  • To enable document properties (e.g. title, description, author, etc.) to auto-fill into SharePoint columns when the documents are uploaded, please take a look here.
  • To automate the complete migration process for a large number of user home drives to OneDrive for Business by scripting (including user provisioning and other required steps) take a look here.
  • You will have all features of a native SharePoint library or list, including change notifications or workflows on external data change.

 For more information and a free download of the Layer2 Cloud Connector see here.

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