Oh... I completely forgot another issue.
After replacing HDD, you might get a Wifi error. If you do, you also won't be able to downgrade via recovery mode, only reinstall firmware or upgrade via XMB (which would be not wise to do on CFW 4.xx).
You might be able to downgrade by unplugging the Wifi/BT module off the PS3 and use a noBT firmware (controller will have to be plugged via USB, obviously).
PS3 Slim CECH-3004A 160GB (500GB)
PS2 Slim SCPH-70004 - FMCB 1.8b