You want the same model but make sure they are from different batches.

When buying SANs the vendor will routinely mix the disks supplied to minimise the chance of any manufacturing defects taking out multiple drivers at once

Yeah I should have been more specific there. For RAID you probably don't want the exact same batch for all drives, accident waiting to happen.

Now CD/DVD-Burners on the other hand, if you ever want to run 4 of those at once, at the shop we found your best bet was to get sequential serial numbers, back then even a firmware rev could make a multi-burner setup squirrely.

But I suppose that's a different form of Array...

Same drives, good; same batch, bad.