That error message is almost certainly nothing to do with the 6.1.9 upgrade, but is instead an issue with the USB stick.
I would suggest that you start by putting the drive into a Windows computer so that it can check (and possibly repair) the file system. Then extract the bzimage and bzroot files from the ZIP download of unRAID 6.1.9 and overwrite the versions on the USB stick with the extracted version. If that does not work then I suspect that either your USB stick or the USB port is starting to fail. While you are at it I would suggest making a backup of the USB stick files to a location on your PC.
Update. Yep, was the flash drive. Would not even format correctly. Surprised that it died that quickly.... Used an 8GB that I had, did a fresh install of 6.1.9, copied the config folder, did a replace key, and then brought the whole thing back online. Had a full backup of the config folder just in case the old files were junk, didn't need to use. Thanks for the above, went into full panic mode right off the bat.... ><
-e-