ES40 won't boot after disk fw update

Hi,
 
I've got an ES40 running 5.1b. I've just updated the drive firmware and now the system won't boot. It fails with a message "kernel stack invalid, halt code =2 ". Boot block loads ok before the crash
 
For info there's another drive in the internal bay which holds a linux installation and it won't boot either, so it seems the firmware update is definitely the issue.
 
Any ideas of how this can be fixed? I can't easily downgrade the firmware as I don't have a record of the old version (Doh!).
 
Cheers
 
Joe