This happened to me with my Xp50 when I had one. I would usually go back an initiallize everything as DN instructed.
On the other hand, this doens't always work and is a very serious problem.
If your XP50 has a serial number between 0 and 25000, then your XP50 has Flash ROM. You can download an OS upgrade from the Roland Denmark site (
www.roland.dk). Just copy the downloaded file to a blank floppy, put it in the disk drive and then turn onthe XP50, the XP50 will write the OS and repair itself.
The Infamous EPU.