Confused about fsck

Marius Gedminas marius at
Wed Dec 14 16:14:20 UTC 2011

On Wed, Dec 14, 2011 at 04:01:16PM +0000, Colin Law wrote:
> Every so often my system checks my disk on bootup.  I thought that
> this was done using fsck.


> I have an external 250GB USB disk that I wish to check.  I plug in the
> disk and check it is not mounted (it appears as /dev/sdb1) and run
> sudo fsck /dev/sdb1
> This returns immediately saying that the disk is clean, whereas I had
> expected it to take a little while checking things, so it does not
> seem to be doing the same things that the check on startup does on the
> built in disk.

It is doing the same thing fsck is doing to your built-in disk on every
boot: checking if the filesystem needs to be checked.

If the filesystem was not cleanly unmounted (e.g. you unplug the
external disk without first unmounting it), fsck will check it.

Otherwise it will see if the filesystem has been mounted X times since
last check (or, alternatively, if more than Y days have passed since the
last check).  This is why your computer checks your system disk "every
so often".

Otherwise it will tell you the filesystem is clean and exit.

> I can't see any options on fsck to specify what is to
> be checked.  Can someone explain what is going on?

Now if you want to do a full check, in case you don't trust the
'filesystem clean' bit, you can force a check with

  sudo fsck -f /dev/sdb1

If you want to check for physical problems with the disk, use smartctl
(or the awesome GUI tool Palimpsest) instead.

Marius Gedminas
MSDOS didn't get as bad as it is overnight -- it took over ten years
of careful development.
                -- dmeggins at
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 190 bytes
Desc: Digital signature
URL: <>

More information about the ubuntu-users mailing list