According to Tru64 man page (later version 5.1) you should see the drives:
The second example specifies device names in the command:
# iostat dsk2 dsk3 cdrom2
tty dsk2 cdrom2 dsk3 cpu
tin tout bps tps bps tps bps tps us ni sy id
0 13 11 5 5 2 2427 1213 0 1 1 98
Found at
Hp-Compaq-Tru64 site