What are you attempting to achieve ?
Just the actual data or the 'meta-data' too ?
There is always the 'rm' command :)
If you want/need to clear all, rm -r (the directory), and then use the 'mktape' command to recreate.
If you want to clear just the data, simply rewind the tape and write a single block of data.
(mt -f /dev/nstXX rew; dd if=/dev/zero of=/dev/nstXX bs=1k count=1)
Regards from Australia
Mark Harvey