I also had a failure to recognize a new drive letter. To fix it, first I tried going through the steps to rename the drive from the new letter to the old one that ST was accustomed to seeing. When I tried to run the sync, though, even then it
refused to run the sync. The path was exactly correct, the drive was definitely relettered, visible, and browsable, but I got the same error message (Failed to execute folder pair / could not locate folder). I wonder if there was any possibility
there was a driver assigned to that path from before if there was an improper hardware removal in past, I don't know.
I'm not sure why the drive letter was different, I thought the situation was identical to before, not sure, but I think I got around it by doing a full power shutdown and restart. Still, the drive letter recognition feature does not seem to be failproof.