Do such enclosures exist? And if so, are there any that are usb powered and don't need an external power source? I ask because I'm replacing my laptop harddrive with a larger one and I also have another couple of 2.5" external drives lying around from before and wanted to combine them.
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This sounds like a small NAS device meant for RAID1 specifically (data mirroring for reliability). Most are 3.5" but I imagine there's 2.5" ones. I suspect the cost is high and that you should stick with a single USB drive deal.
Single 2.5" USB:
2.5" USB drives do not require power. You can simply buy a 2.5" USB case. You may wish to get one compatible with eSATA but that's not really a big deal and adds to the price.
USB drives are now quite cheap now and a new USB hard drive might not cost much more than a USB case.