It's hard to tell. I bought 2 Western Digital passport drives for it (usb 3.0 compatible) and gave up on them as they kept giving me problems. I tried to disable their spindown, but it didn't work. They have a hidden partition which can be disabled, but can't be removed. If you connect them to a pc, they also keep asking for a special driver, even if you have their security disabled.
I bought a Packard Bell 2.5 inch external drive, as I had good experience with some 3.5 inch packard bell drives. The thing also spindowns after a while.
I finally bought some cheap eminent 2.5 inch usb enclosures and some western digital laptop sata drives for it. They work like a charme. Never any issues with them.
The advantage from this approach is that you might be able to return the enclosure if it doesn't work.
They might be a little more expensive in a trusted retail computer shop, but for that reason, it's worth it.
If you use a laptop harddrive in an usb powered enclosure, make sure that the drive doesn't need much more than 0.5 amps on it's 5V.
This is the theoretical maximum current you are allowed to consume from an usb port. Mine is 0.55A which still seems to work as well.
Are you sure your drive has a 512 byte sector size, and it's partitionned and formatted with a 32KB cluster size? This is the most common error
for harddisks working with usb loaders but not with neek2o.