File system bafflement

[Paul Smith]

On Tue, 2014-09-02 at 21:14 +0100, R Kimber wrote:
> No I've never personally done that, though something clearly has.
> However, what you say is broadly correct, but I think it was something
> to do with the failing disk that I mentioned. This was a USB backup
> disk mounted at /media/zip and has worked properly for a very long
> time using a bash script and rsync. But in the last couple of days the
> backups have failed and the data seems to have been written
> to /media/zip on the root partition instead.

Sorry, I missed the first few messages.  There is no way, short of a bug
in the kernel, that anything can write data "behind" a mounted
partition.

If I understand you correctly, it sounds like the USB backup disk became
unmounted somehow then was mounted again before you noticed.

That would explain the problems: first the backup disk is unmounted (by
hand or USB unplugged or powered down or something) so the bash script
was backing up files to a directory it thought was the backup disk but
was really the root partition, filling it up.  When that happened the
backup failed because it ran out of space.  Then somehow the backup disk
partition was re-mounted, either by hand or it was plugged back in to
the USB port or whatever, and that "hid" the old backup files.

You can change your scripting to ensure that the partition being used
for backups is really a separate partition, not root, if you want to
avoid this kind of failure in the future.