We have received a number of unconfirmed reports that backing up QuickBooks files is not straightforward. If correct, QuickBooks files would not be the only ones with this issue. Nearly all databases need to be closed before they can be backed up without corruption, and it is our understanding that QuickBooks files are basically databases. So while we don't have confirmation of this, it is our understanding that a QuickBooks file backed up while open will be unrecoverable.
One user reported that "QuickBooks has a .qbw file which is the normal company file that updates frequently, and a .qbb file that is a QuickBooks backup file. I would assume the .qbw file type may not load correctly from a backup but the .qbb file I would assume would work just fine... I have been having QuickBooks make an automatic backup every third time I close the company file. I have it save that backup to a folder which is backed up by SpiderOak. So far, so good."
Another helpful user recommends this QuickBooks support article on how to properly back up QuickBooks.
Although QuickBooks files can be backed up as described above, they should not be synced.