mount /dev/uba1 /mnt/usb
Slackware uses uba instead of sda. Atleast it does on my two systems.
I forgot to mention a couple things that came to mind. Do you have a directory for USB under /mnt? Also, do you have a line in your fstab for your USB drive? I went one step further and have several lines in my fstab for USB drives and I also have several directories under /mnt such as: usb1, usb2, usb3. The /dev files are uba1, uba2, uba3, etc. I sometimes have multiple usb drives connected, at the same time, which requires the seperate directories and fstab entries.