be carefully what Backup you want to flash...
backup of a normal original firmware doesn't WORK
backup of a CFW 3.55 with the no check patch WORKS
backup of a original firmware updated from the CFW 3.55 with the no check patch WORKS
and for the semi dual boot you must patch the backup of your CFW 3.55 as in the tut explain!
who have flash a normal backup without the no check patch use the backup and patch it to make a downgrade bin named as bkpps3.bin!
I hope this will bring light in the problem!!!!
read the tut carefully, when you use your downgrade dongle and the ps3 beeps 3 time you must put the lv2diag and the PS3UPDAT.PUP to a USB Stick and so on...!