A bidirectional sync on Drive to a transfer server fails and shows a static progress bar or displays one of the following errors:
Failed to start sync: Peer error: disk I/O error (code 45078)
Warning Sync session error: [PEER] DB Error. (code 45078)
This error is most likely due to the transfer server running out of disk space.
Check the system that the transfer server is on, and take the appropriate actions to make disk space available.