fsck fails with "partition in use" error after partition umount'ed

[Joel Roth <joelz@pobox.com>]

I notice that /dev/sdb1, an ext4 partition on a USB drive has remounted
read-only.

I try 

umount /dev/sdb1

then 

fsck /dev/sdb1

fsck from util-linux 2.25.2
e2fsck 1.42.12 (29-Aug-2014)
/dev/sdb1 is in use.
e2fsck: Cannot continue, aborting.

Is there a way that a volume can be in use without being
mounted? I don't have a desktop environment, so it's
only vanilla (sid) here.

I suppose I should investigate lsof. I also wonder, 
if changes in udev could be involved.

The underlying issue is that the driver detects an I/O
error. 

Regards,

Joel


-- 
Joel Roth