I was in 6.11 but I was having overheating issues due to bad CPU fan. Held off on upgrading due to this. Server ran fine other than occasional freezes due to temp. Replaced fan. Backed up with CA backup to external drive. Updated. Checksum was valid. Notification said I had to reboot. I get to the selection screen to choose OS, OS with GUI, safemode, memtest. Regardless of what is selected it very quickly flashes "invalid argument" and nothing else happens. I remain at that selection screen. Please help?

Try recreating the flash drive, first backup the current one, then recreate it using the USB tool, confirm the flash drive boots, and if yes, restore only the /config folder from the backup overwriting any existing files.