Switching HDD on cfw and ofw...

So, before I ask anything, this info is needed to understand the dilemma:

I've got two ps3 slims;

* Ps3 2004B (Rogero 4.40 v1.02) with 250gb HDD

* Ps3 25xxx (Ofw +4.xx) with 320gb HDD

My question now is, how can I switch the HDD between these two consoles? Just switching won't work...

I've also got a spare 250gb HDD on the side, just in case..


All help is appreciated

Thanks in advance!