I thought you were gonna stick with the UMS version a while longer Uncle B
Judging from what I've read (having not experienced your particlular problem) it seems very important to not have your player full of music when you try to upgrade/crossgrade the firmware.
When you changed back to the IMM formware, did the firmware upgrade proceed OK (ie the player churned away for a while then reset/reformatted itself?), or did your "FILE CHECK ERROR" occur before this? Is it possible to communicate with the player via your Manager software (in which case you might be able to manually format or revive it)?
Good luck, I hope you don't have to return your toy!