Tar - tape capacity much less than expected

[Stairforce]

I set up my Linux server intending to use it as a back-up server. It has a DDS3 tape drive
on it and the dds3 tapes have a native (uncompressed) capacity of 12GB. I have only just
got writing to tape to work. I have tried using the back-up utility built into Mandriva, but in all honesty it does not meet my needs. I have learned how to use tar instead.

I packaged up the files i want to archive into directories of just under 12GB each and set the
first one to save. It reported
tar: /mnt/st0: Cannot write: No space left on device
tar: Error is not recoverable: exiting now
after writing about 2.75GB. It was a new tape and it was at the beginning. I do not understand
why it reports that the tape is full with less than a quarter of the stated capacity. The tape was at the end when I got it out and looked at it, though.

[Stairforce]

Oops - ignore this - I got my /dev and my /mnt mixed up.