I'm suddenly seeing some strange behaviour on our USB backup drive that doesn't make sense to me:

(2013-10-21 14:58:23 [root@newdc /]$ cd /mnt/backup/
(2013-10-21 14:59:03 [root@newdc backup]$ ls -la
ls: reading directory .: Input/output error
total 0
(2013-10-21 14:59:05 [root@newdc backup]$ df -h /mnt/backup
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda1             917G  843G   28G  97% /mnt/backup

How is it possible for the OS to know how much is in use, but I can't ls any of it as root? Or more to the point, what problem does this indicate?

/var/log/messages said this:

Oct 21 14:57:54 g5 kernel: EXT4-fs error (device sda1): ext4_journal_start_sb: Detected aborted journal
Oct 21 14:57:54 g5 kernel: EXT4-fs (sda1): Remounting filesystem read-only

But...read-only is something different than 'throw an io error'...

After unmounting to try fsck on it, I had someone on-site look at it, and the drive was not spun up, and had a slow-flashing light, which I believe means it was in a power-suspend mode. So I had them unplug and replug the USB cable, and now (before remounting) it says:

fsck from util-linux-ng 2.17.2
e2fsck 1.41.12 (17-May-2010)
/dev/sda1: clean, 2805106/61046784 files, 181934167/244182016 blocks

I then mount it and now ls works and df reports:

Filesystem            Size  Used Avail Use% Mounted on
/dev/sda1             917G  680G  191G  79% /mnt/backup

What would cause it to go into such a state without being asked to? Why all the weird behaviour, and now it appears to not be corrupt?