The following command should mount the device if possible:
# mount -t vfat /dev/sda1 mnt/usbdrive
Reasons the above command may not work:
1) You are not logged on as root. Type the command "su" and give root's password. This won't work in ubuntu, instead use sudo and give YOUR password:
# sudo mount -t vfat /dev/sda1 mnt/usbdrive
2) You haven't created the mountpoint. Chances are you haven't got /mnt/usbdrive as I made that up. You can create the mountpoint with:
# mkdir /mnt/usbdrive
or substitute /mnt/usbdrive for where you want to mount it.
3) You have a SCSI drive already in your system. The thumb drive will take the first unused SCSI device so if you already have one drive it will be /dev/sdb1 instead.
4) The drive might not be formatted to vfat (FAT32), try "-t usbfs" instead or maybe "-t auto" will work.
5) You do not have the correct USB modules loaded, what is the output of "/sbin/lsmod" after plugging in the drive? Have a look towards the end of the output of the command "dmesg" to see what is happening when intserting the device. Try pluging the drive in and out whilst running the following command:
# tail -f /var/log/messages
6) Your kernel does not support USB. I don't know about DSL but both knoppix and ubuntu most definitely support it (I have used both myself with USB devices).
7) Your drive is linux-unfriendly. I have come across two thumbdrives I could not get to work under linux. With the first one I could only mount a very small partition which contained what looked like driver files which would enable a windoze machine to view the rest of the drive. The other one just plain wouldn't work. It wasn't mine and I couldn't be bothered to find out why.
8) Another reason I can't think of right now!
What make and model is it? Have you googled for it and linux?