Exactly. Even if it ended up being the wrong kind of drive, you could get a more appropriate drive and copy the bare-metal backup onto it. That should work quite fine. You can get USB SATA cradles which you pop SATA drives into like bread into a toaster(hence the name "hard drive toaster"), but if you're just going to have the same drive sitting in it all the time, a case would make more sense and look better(no exposed electronics when working, no carrying drives around in static baggies). You can't be 100% certain without opening your server's case, but given the size and age of the disk I think it's probably SATA. It's getting harder and harder to find old-fashioned wide cable ATA drives these days.

Look for a USB enclosure with a removable plate in the front for accommodating a CDROM drive, those are intended to be opened and will definitely have a standard disk in it. Little handheld USB hard drives will only have laptop-sized drives in them at best -- sometimes you get a weird disk that's not even laptop form-factor.

Don't get a drive that's only big "enough", get a drive that's plainly bigger than what the server has. You don't want to take a chance on an "identical" drive actually being smaller because of the different ways of measuring capacity(mebibytes vs megabytes).