opening the Mega case is a bit difficult indeed because there's a plastic rim running under the disk drive that is hard to get out. You can jerk it free eventually without breaking the plastic casing with patience and care.
I'm not sure whether unscrewing the disk driver helps because the plastic rim is potentially under the shielding around the disk drive, i can't really remember.
But the method mentioned first is how i got my MegaSTs dismantled without breaking the plastic and even got them back together.