Hardware
I really dislike hardware. It's fiddly, it's easy to hurt yourself when upgrading it, it doesn't come with source code, and sometimes it can be figure difficult to figure out what's going wrong.

In our particular scenario, our mobile training server occasionally forgets about its hard-drive entirely, independent of operating system. It's the good old 'lost interrupt' message, and when it happens, not even the BIOS can find the missing drive.

We replaced the hard-drive but the problem continues. Remaining suspects are the cable and the IDE controller. We've swapped the drive to both a new controller and a new cable, hoping that will make it stable. We really want it to be stable, as we're running a course tomorrow and a dozen people will be using it, and wanting the hard-drive to remaining available while they do so.

Close inspection of the removed cable reveals a small nick on one part, so I'm hoping it's just the cable. Dying IDE controller means we get to replace the motherboard, and this is a rather non-standard motherboard in a rather non-standard case.