I own an HP ProLiant MicroServer, a great little box I bought a couple of years ago to act as my main file server/NAS machine. It's held up very well and it was very cheap because I got £100 cashback in a deal (and it was cheap already).
It's not a powerful computing machine by any stretch but a very decent server: I've put 8GB RAM in it and 4 2TB disks in RAID5. It's also very decently built by HP, with some care and attention you'd except on a bigger server. Hence the Proliant badge.
One reason I prefer it to my QNAP T419P is that it's got a display connector. The QNAP is serial only, so a bit more fiddly.
To maximise the available storage capacity, I installed Debian on an 8GB USB stick and use the 4 hard disks for the RAID only. Generally, this has been fine, but I have started noticing some fairly severe I/O latency hits recently and this has started causing more frequenet pain elsewhere. Combine this with some USB filesystem corruption a few weeks ago and I wanted to switch away from this configuration.
However, I also learned that the stock HP BIOS does not enable all the system features, including a "spare" SATA port on the motherboard, supposed to be used for a DVD or CDROM. Without another SATA port, it's impossible to add another drive for the OS.
Luckily, I came across a great web page by Joe Miner describing how to update the HP BIOS and enable these hidden features. The usual caveats apply: this is not an officially sanctioned "update" (in fact, it isn't adding anything, but "un-hiding" things. The version remains the same).
Having done the update, I now have an extra motherboard SATA port and have also made all the ports default to 3Gps. I've also stuck a spare 2.5" SATA hard drive in the empty CDROM space.
With this extra disk installed, I used debootstrap to install a new version of Debian on the disk and configured this new install, adding boot loader etc., while the "old" system was running. On Sunday morning I rebooted into the new system, fixed up a few missing bits and pieces and now have a brand new OS installed on a proper disk. So far, so good.