When you got to the partitioning screen, were there one or two tabs toward the top left? I have two physical drives here, and each has its own tab, one for hda, and the other for hdb. For your case, the first (internal) drive will have a tab labeled sda and the second (external USB) drive should have a tab labeled sdb. The first (internal) drive will be displayed by default. To see the second drives information you must click the sdb tab to bring it forward.
I do not use USB drives here, so I do not have any direct experience to pass along, but you could get a copy of Mandriva One (an installable live CD) and boot from it to see if the USB drive is listed in 'MCC / Mount Points / Create, delete, and resize hard disk partitions' (looks just like the partitioning screen in the installer). If you see the drive, close MCC and click the Live Install icon on the desktop to start the Mandriva One Install Wizard.