Ok,
I Experienced this problems before. You may try with this steps.
If you can tar cvf /dev/rStp0 .(or rStp1 if this was the second drive), but cannot do "tape" commands, then run mkdev tape again and select the default tape drive. Or for a quicker test:

rm /dev/xct0
mknod /dev/xct0 c 46 128 (assuming rStp0)

and then try "tape status" etc.

For tapes, the minor number for ioctl is 128. The ioctl devices are /dev/xct0, /dev/xStp0, etc. If you have created a SCSI tape, but didn't set it as default, then the /dev/xct0 is

crw-r--r-- 1 root sys 10,128 Sep 21 10:35 /dev/xct0

which has the right minor number, but the major (10) is for a cartridge tape, not SCSI.

You could fix this as suggested above, or by editing /etc/default/tape and changing the

device = /dev/xct0
to
device = /dev/xStp0

Give a try first, let me know if this helps.
Works for me.