I had some problem with mount / umount pendrive devices.
I am used to do:
mount /dev/sda1 /mnt/pendrive
as su. Usually the first time I can mount successfully my pendrive device, and the second time a cannot. Obviously I umounted the device before remount it again.
Tipical erros I give are:
mount: No medium found
mount: you have to specify file system type
ps: I'm using RH9 (kernel 2.4.20-8)