lvm and unmounting lv

Hi list;
I'm new to lvm and had a few questions:
- after adding a hd (hdb) and configured/partitioned , creating a gv for hdb1 and hdb2, and creating 2 lv's on hdb2 i've always to 'fsck' the 2 lv's.
Apparantly they are not unmounted when system halted.
error messages :
"removing gv_hdb-lv_res"
"Unable to unlink device node for ' gv_hdb-lv_res' "

What happen exactly ?
Thanks for your help in advance

mess-mate