you have to downgrade back to 3.55 use the 999 downgrader, you may need to get more info on how to change hard drives on 4.21 cfw, also i personally wouldn't touch segate hard drives with a barge pole, it's cheap for a reason, get western digital or samsung drives, they may be more expensive but your data won't be so much at risk with those brands.
PS3 Slim v3.55.4 CFW Rebug 1TB Internal, External 2TB
XBOX 1 with XBMC
Wii 3.2E + Cfg UsbLoader External 1TB
DS lite & DSiXL + Cyclo ds 8gb micro sd DStwo 16gb micro gd
PS2 V14 legit matrix infinity modchip
PSP slim 2006 5.00 M33-6 8gb memory stick pro duo
PSPgo with 16GB M2 6.60 PRO B10