problem with lvm on an usb drive

[Sébastien Barthélemy]

Hello everybody,

I have a external usb disk with a single LVM partition on it.

When I plug it (on my ubuntu hardy computer), /dev/mapper is populated
automatically and I can mount it without any problem. However, when I
unmount it, unplug it, and then plug it again, I cannot mount it
anymore (until I reboot the computer). It says the partition seems
empty:

$ LANG=C sudo fsck /dev/mapper/backup-backup
fsck 1.40.8 (13-Mar-2008)
e2fsck 1.40.8 (13-Mar-2008)
fsck.ext2: Attempt to read block from filesystem resulted in short
read while trying to open /dev/mapper/backup-backup
Could this be a zero-length partition?

I guess there is some housekeeping not taking place. But I don't know
what. I'm not really aware of how udev and the device mapper work,
thus I don't know where to look, and for what.

Could you help me understand what happens ? A link to the good
documentation would be of great help.

Thank you in advance

cheers

-- 
Sébastien Barthélemy