Issue occurs when attempting to copy data from server to attached USB3 drive.
The problem is apparent when copying a directory with more than 100 files (movies).
If manually selecting the individual files, and ONLY selecting 100 files, then the process completes.
If selecting the actual drive that contains more than 100 files, then the process will start as expected, appear to be processing the data with a refresh of the app on that drive, but will eventually fail with the stated error and not have completed any file transfer.
I do not know if this is a limitation of file caching for this process and have not noticed if the process exceeded the 30 minute maximum sleep mode setting for the server, or if there is a legitimate limit on data size or selected object count for file transfer.
The “WorkAround” is to manually perform the file transfer by selecting each individual file to be transferred and not to exceed what appears to be a maximum limit of 100 files, regardless of size.
Possibly this can be forwarded to WD Support to improve the operation, adjust/allow for a longer sleep mode setting or identify if there are limits imposed based on standard operations and activities.