It looks like a fairly standard 5V/12V connector, however I had a customer substitute a powersupply on an enclosure once.
I told him to back up to DVD before we moved from his old Mac to his new one, but he brought the machine in and said "Oh man, those DVD's, they take 20 minutes each, I would've been there for hours! So I didn't bother." We backed up all his work to a new Seagate hard drive (in an old enclosure, 'coz it saved $40) and he formats his old drives. Now, this new drive is the drive that he had stored his entire life's work on (photoshop files... can you see where this one is going?)
He gets home with it, and can't remember which power supply goes with the enclosure, so plugs in the first one that fits -- and it burned hell out of the brand-new Seagate drive - toasted a couple of components right off the board.
Fortunately, we were able to buy another Seagate drive of the exact same model, and I swapped the circuit boards from the new drive to the old one, the toasted drive spun up fine and I recovered all of the data onto yet another new drive.
So where are we, crazy -- if this is an external hard drive, it'll cost you a bunch of searching and about $35 with shipping to order a replacement AC adaptor. Or you can get a whole MacAlly external enclosure for around $45. (if it's a CD/DVD it;ll cost a little more.) So...