This fix, done exactly as the OP (@Foxglove) did, DID NOT resolve the issue for me. I ended up using the recovery procedure here => https://wiki.unraid.net/Manual/Changing_The_Flash_Device#Using_the_Flash_Creator
To rollback to a backup of 6.9.2 from earlier in the week.
FWIW, the currently advertised update of 6.10.1 (As of 31MAY22) still does NOT have the added
syslinux.cfg line noted in this and the other (https://forums.unraid.net/topic/119502-bzimage-checksum-error/) forum posting.
I would highly suggest the update stop being advertised for normal
customers until this is resolved (I only say this since this is a paid product and not FOSS).