DDS3 not backing up full amount

[Brandon Hagedorn]

Have a question about dds3 tape drives... I have a SONY SDT-9000 and
when I use this command "tar c backup/ --totals" I get this message when
the backup contains around 11.4 GB of files, here is the message 
"Total Bytes written 10612029440 (9.9GB, 994Kb/s)   tar:/dev/sa0: Wrote
only 0 of 10240 bytes     tar: error is not recoverable: exiting now"  

Was wondering why it's only going to 9.9 GB when the tapes are 12/24GB?
I've tried 4 different (new) tapes and tried backing up 40GB of data (
which I knew it wouldn't backup the whole thing). The same thing
happens, stops at the 9.9GB point.

Any suggestions?

Thanks, Brandon