Anytime you plug a drive into a system it always makes you format it. Not sure if they've changed that with a system update or not, but I don't know if there's a way to just use an old drive in a new system without having to format it first. (obviously that's gonna kill all your saves)
There is a way. I think you have to have a jailbroken PS3 to put it into, but you could always "unbreak" it after you swap your HDD. Link
Haven't tried it yet. My PS3 is dead, but I don't have $$$ for a new one.
My advice would be to do the hair dryer trick to resuscitate it long enough to back up your data to an external drive. otherwise, you wont be able to plug in the drive without having to wipe it.