Can an admin of the mailing list please delete these posts with the personal private information.
Anyone replying to this topic, please remove the quoted text with the private info.
Michael, please be careful when you post things like this.
As for the problem, I really think you would be better off doing your backups to an external hard drive, flash drives will not last as long as a hard drive, and you seem to have very bad luck with flash drives.
As for what you are backing up I believe you want to save all your photography, so I'm not sure you need to be backing up the source for darktable everytime, as that is already available on the net.
An lastly I think you should use the --delete flag not the --delete-before
If you delete all the files before the copying starts then if the copy stops at any point those backups are gone, yes they will still be gone after full Sync runs, but if you delete a directory by mistake on the original, during the copy you might notice it's deleting something you don't want to lose and you can kill it before it deletes everything from that directory and recover whatever is still on the backup drive.
I'm not sure you are gaining much with the --delete-before flag