why would you ever unplug a hard drive with the computer on/plugged in?I thought RAID 5 would keep working unless you lost two drives, did you?
edit:
I assume you have four drives? Why not just RAID-10?
RAID10 is a waste of hard drives.
I found out what happened, and it makes me pissed at myself..
I have 3 hard drives. They are all cooled, but the graphics card spits out hot air into one of them, and since today is so hot the hard drive overheated as I was doing hard drive intensive work. And it shut off. That is why it disappeared. So I went to unplug it, and plug it back in to restart it. But I unplugged the wrong one. Then I had a completely degraded raid5 with only one drive still active in the array. No big deal though, I have all the raid tools in ram still, I can fix it. Right? And at this time, rebooting is a bad idea because the system wouldn't come back up. So I went to reboot the drive that was the original problem, but it is on one of the cables on the PSU which are really crappy and it somehow shorted and shut off the system. GRRRR
I actually have a box of high quality SATA power connectors that I just swapped in, so this wouldn't happen again.
So anyway, I found out a way to fix it. Fortunately the first partition on the first drive is a stand-alone partition with /boot on it, the files responsible for loading up the initrd, and ultimately the linux kernel. So I created a custom initrd on my laptop with the raid tools, put it on the drive via a USB to SATA adapter, and booted to the custom initrd. Now I'm working on rebuilding the raid from that.
At least I know WTF I'm doing, otherwise this would be a real pain