Thanks. I had a look at the HP but their webpages are aimed at professionals and I don't believe an OS was mentioned once, hence the question.
I understand completely. No worries! They don't ship them with an OS (most people probably buy them for Linux) but you
can put Windows on it, if you buy it separately.
I like to conserve energy and switch everything off (we have a single switch which will do that) when not in use, but then MC would no longer be available.
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There is also a structural dimension to this. Since MC depends for most of its main functionality on data residing on hard disks, it stands to reason to integrate it with a server.
Yes. I understand this too (and had originally assumed you were going for something like this). In this case, the only real option would be to run a "real" server. Now, it is worth noting that the particular HP I noted, or any similar box, is useful primarily because it is:
* An actual server with server-class hardware (always on rated, error correcting RAM, etc).
* In a rackmount case, which was one of your requirements
* Has a bunch of nice hot-swap hard drive bays on the front of the unit.
There's no reason you couldn't do it with something smaller and cheaper. You could use something like a NUC connected to a USB hard drive array box. Or a cheap PC (perhaps home-built) in a regular tower box. Or even build a server-style rackmount system yourself in a Norco or iStarUSA rack-mount case (though iStarUSA typically doesn't have very nice hot-swap bays, and Norcos have awful loud fans, so be a bit wary), and then save money by using a consumer-class CPU, RAM, and motherboard in the system.