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)