reading dat tapes

Hi,
I've got a few tapes (2gb) made with retrospect, a software for apple systems. 
I'd like to read all of them and put on a disk or a few dvds, thus I tried to 
use my linux dat (20gb) with the command:
dd if=/dev/st0 of=/tmp/dat1 bs=1024
but it doesn't copy anything (exits with 0 bytes read and 0 write). Am I doing 
something wrong or just Linux cannot read retrospect tapes (I guess it 
should, being a raw copy)? Or simply I cannot read tapes of 2 gb with a 20gb 
reader?
Any idea?

Thanks,
Luca
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