I found this answer on a forum (link below):
* take out the hard drive
* connect hard drive to pc
* open up disk management - in win7 it asked me to make a master book record, so I did.
* format in NTFS - that's right, NTFS - don't use quick format
* get the rom of whatever version you were on before you upgraded - i had to search, but i found the us 3.30 rom
* copy that rom to usb drive in folders PS3 ==> UPDATE ==> put rom named PS3UPDAT.PUP here
* shut down pc, take out hd, take usb drive too
* put hd back in ps3, put usb stick in ps3
* boot ps3
* a screen will pop up. press select + start
* another screen (i think) later, press select + start
* configure your ps3 setup - done
(source: http://forums.demigodthegame.com/386771)