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You can "roll your own" external hard drive for less money than buying one off the shelf. In addition to saving money, you can get some very useful backup software & have the size & specs you truly want - not just what's available at the moment.For a solid backup dirve on a budget:This enclosure + This drive = 250GB external drive for ~ $115 with some good software.Compare to this one from BestBuy that costs $25 more, has no fan, and no good backup software. I have a fanless, mostly plastic external enclosure on my other drive. It does what it should, but its very hot. I'd recommend going with aluminum and a small fan in the enclosure. You'll pay a little more, but you could also extend the drive's useful life a few years.
Is there an advantage to having an external drive versus having a slave drive installed within the case for backups?
Seagate, Western Digital, whatever. It's all the same.