[SOLVED] Mounted USB stick immediately unmounts itself - why?

Question

Hi all,
I thought I was unable to mount a USB stick, however it does actually mount but then it gets immediately unmounted by something:
# mount /dev/sdd1 /mnt
# ls /mnt
<empty folder>
# umount /mnt
umount: /mnt: not mounted
# mount /dev/sdd1 /mnt && ls /mnt
<contents of USB stick>
# ls /mnt
<empty folder>
Somehow the USB stick is getting automatically *unmounted* almost immediately after I mount it!
I don't have any automounts enabled, so I'm not sure why this would happen.  If I make a different folder (e.g. /root/test) and mount it there then it sticks, it is only the /mnt folder that gets automatically unmounted.
Any idea what's going on?  I've never seen this happen before, and last time I tried this (a week or two ago) it worked fine.
Last edited by Malvineous (2015-03-15 01:15:16)

Answer 1

brebs wrote:Use "mount -v" (i.e. "verbose") to get some feedback - it's most likely not being mounted.
Like I said in my first post, if you run "ls" immediately after the mount then you can see the contents of the USB stick, so it's definitely getting mounted, but waiting only a few hundred milliseconds after the mount reveals it has been unmounted again.  What is causing the unmount is unknown.
brebs wrote:
Immediately after running that mount command, run:
echo $?
To see the return code. It should be 0.
Yes it is zero, because the mount command succeeds.  "ls" wouldn't be able to list the files on the USB stick if the mount failed.  It is something else that is unmounting the folder mere milliseconds after it is successfully mounted, but I don't know where else to look to find out what it could be.
brebs wrote:Also, mounting to /mnt is very weird - you should be mounting a directory *inside* /mnt.
I am mounting inside /mnt, I just used /mnt as an example to keep things simple.
Vain wrote:
I made the same “mistake”. Try running “systemctl status mnt.mount” and see which device the output refers to. For me, it still listed the old device name. Hence, when you mount it, systemd tries to find the (old) device—which it can’t—and thus unmounts your device immediately.
I was too lazy/annoyed to find out how to solve this properly. I rebooted my system.
Interesting idea!  Unfortunately I have nothing listed for this particular mount point.  The only *.mount services on my system are the ones that are mounted at boot (e.g. srv.mount for /srv).  I have the "noauto" option against this mount point in fstab (as USB sticks aren't normally attached to my system when I boot) so sensibly enough I don't seem to have a mount service running for this particular mount point.