How do I get df to work on jump drive?

[Knute Johnson]

I've got a jump drive that I use for nightly backup.  I can't seem to 
get an accurate space left or used.  The disk is /dev/sdb1 and the mount 
is /media/usbdisk.

knute at knute4:~$ df -a
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/sda1            301717936   7192208 279199320   3% /
tmpfs                  1031008         0   1031008   0% /lib/init/rw
/proc                        0         0         0   -  /proc
sysfs                        0         0         0   -  /sys
varrun                 1031008      1000   1030008   1% /var/run
varlock                1031008         0   1031008   0% /var/lock
udev                   1031008      2812   1028196   1% /dev
tmpfs                  1031008         0   1031008   0% /dev/shm
devpts                       0         0         0   -  /dev/pts
fusectl                      0         0         0   - 
/sys/fs/fuse/connections
securityfs                   0         0         0   -  /sys/kernel/security
knute at knute4:~$

thanks very much.

-- 

Knute Johnson

"The urge to save humanity is almost always a false front for the urge 
to rule."

   H.L. Mencken