Device already mounted or mountpoint busy

First post in too long. The muse hasn't really grabbed me strongly enough to warrant additional entertainment for my tiny readership. Still, I won't give it up!

So here's a quickie technical post. I powered on my Ubuntu 9.10-powered computer the other day. When I logged in, I found that my home directory didn't exist, and was dropped into the root directory.

I mounted it manually, and everything looked okay. Assuming it was just some fluke, I rebooted the machine to make sure it would auto-mount correctly. It failed again. This time, manual mounting didn't work.

My home directory actually lives on /dev/md0, which is a Linux software RAID1 (mirror) device. I checked, and the device existed, and the array state was good:

Personalities : [raid1] [linear] [multipath] [raid0] [raid6] [raid5] [raid4] [raid10]
md0 : active raid1 sda1[1] sdb1[0]
      976759936 blocks [2/2] [UU]

      unused devices: 

Still, the manual mount failed:

# mount /dev/md0 /mnt/share
mount: /dev/md0 already mounted or /mnt/share busy

Huh? I ran the mount command with no options: neither /dev/md0 nor /mnt/share was listed anywhere. I ran lsof: neither of those files were listed as open.

I even tried mounting in a new directory:

# mkdir /mnt/tmp
# mount /dev/md0 /mnt/tmp
mount: /dev/md0 already mounted or /mnt/tmp busy

Well, clearly /mnt/tmp isn't busy, as it was just created! So what was wrong with md0?

At this point, I started to get a little worried, so did the sanity test to make sure my data was still there:

# mount -t ext3 /dev/sda1 /mnt/share
mount: /dev/sda1 already mounted or /mnt/share busy

I tried the same thing with /dev/sdb1, the mirror of sda1 in the RAID1. Same result. At this point, I was more than worried, but not yet panicked.

I checked dmesg, to see if there were any obvious problems. Nope. So then I went into /var/log, and started looking at those files for clues. I finally found something:

EXT3-fs warning: mounting fs with errors, running e2fsck is recommended

Ah-hah! So I ran e2fsck on /dev/md0, and corrected the errors. I was then able to manually mount my home directory. I rebooted, again to make sure it came up, and it did.

I don't know if this is considered a mount bug or not, but the error message is extremely misleading.