Check your CF card, might be corrupted or just not compatible. I recommend Kingston, SanDisk, Toshiba or PNY.
Also, if you don't mind losing all your sounds, reformat the card.
If thats not the case, then make sure the 128MB RAM is nice and snugged in the 500. Be delicate with the plastic clips and make sure you hear a click sound, that means its tight in. I was bangin' so hard one day that the RAM popped out a bit and it freezed on me.
If all else fails, then reinstall the OS, you can get it at
http://www.akaipro.com/mpc500 its free.
And if it stills fux up, then sell that shit, but don't destroy something someone else can fix and enjoy. These are instruments.
