NEW I have added some troubleshooting, that I have been throw during my use with usb-disks under Linux.

This howto discribes howto mount a usb-disk on Linux.
This howto is also discribed at linux-usb.org

Steps

1. Make sure that you have SCSI-disk support in your kernel
2. Make sure that you have usb-storage support in your kernel
3. Edit your fstab

none            /proc/bus/usb   usbdevfs defaults,devmode=0666  0       0
/dev/sda3       /mnt/usb_hdd    auto    noauto,user,exec        0       0

4. mount /mnt/usb_hdd
5. Done

Ext3

I have discovered that if the usb-disk your are trying to mount contains an ext3 file system the mount gets very unstable. I fixed that by converting my usb-disk to vfat, since then there has been no problems.

Next you can make a bootable system on your USB-Disk by following this howto .