There is no proper way to do it. You can use the mediainit command but its not necessary and i dont know if it will work successfully on an Exabyte tape.
Just use dd to blast the tape to whatever size you want. Use /dev/zero, if it doesnt exist create it with;
mknod /dev/zero c 3 4
Then use dd;
dd if=/dev/zero of=/dev/rmt/0m bs=1024k count=1000
This will write 1Gb of zeroes to the tape. You can adjust as needed.
Im from Palmerston North, New Zealand, but somehow ended up in London...