NAS, generally is considered a type of "plug-in" file server (oversimplified here).
Power hungry or not, the energy "savings" are not significant enough with newer equipment to offset the COST of the newer equipment. On top of that, with the recent IRS clarification that computer purchases can NOT be expensed but must be capitalized over 3 years you get an additional financial ding.
As for the concerns about hard drives I would have to ask, how many here, in their years in IT have had one fail? Of the ones that did fail, out of how many total hard drives? And how long did it run? It is *very* common for them to last many years with no problems. And that's why you have backup anyway. If you are recycling several computers you can cannibalize them for their hard drives and use RAID for fault tolerance. This way even if something happens to one drive nothing is ever lost.
As for RAID, even WinXP supports software RAID, just not on the OS partition.
You COULD use WinXP as a file server but I would NOT recommend that. Because MS is discontinuing support it could become a serious weak security link. I would look at some of the lightweight Windows Server versions (Win 2008 Standard) or possibly Linux if you are comfortable with that.
Again, with ALL things file server and data related backing up is a critical issue.
One other good use for old PCs is as a print server. For that you could even use WinXP as long as you were not doing data storage. While not impossible it would be difficult to put a malware payload into a print queue that would affect the print server. So WinXP could be used for some time after support is ended if you isolate the usage to a shared print / print server box.
Just some ideas...
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