Sorry to hear that man.
I just had this happen with my CECHA01 and it resulted in my having to send the unit in for repair. My screen would lock up within 30-60 seconds of start up. I couldn't make a backup of my game data to restore later and I was justifiably concerned.
The Sony CSR I spoke with told me I could just pull my 640gb HDD and send in the unit for repair with the 60gb HDD that I still had. I chose repair over replace so I could retain the PS2 compatibility, card reader, 4 USB, etc.
I'm sure most of you see where this is going...
Long story short... got the unit back with a new motherboard and ESN so my old 640GB is serialized to the old board. Good times.
Hopefully you won't have to send it in and you already had your data backed up.
It looks like I might be able to decrypt - merge - re-encrypt the 640 with the new serialized HDD and "restore" my game data but I honestly can't find a definitive answer despite abusing the search function and combing the forum.
If you had to send it in for repair you could even slip a blank drive into it then maybe work out the solution above... IF it actually works.