Hi Ititmpi,
Yah, it was weird, but I do have one update...
To "restore" the usb after upgrading, I simply copied the files from my previous 6.9 backup to replace the files on the usb. I didn't use the restore usb function. I thought I'd try to simply replace the files first.
After that, it booted correctly immediately, but to my surprise, the update to 6.10 survived replacing all the files from the backup. Everything has worked perfect. I guess something just got corrupted in the configuration during upgrade??? I don't know. Sorry! I wish I could give you more debugging specifics, but for anyone else that runs into this problem, simply replacing the usb files from the backup worked like a charm!