reading DEM data

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I was trying to read DEM data from 1/2" tape drive into my SUN account using
"m.dem.extract" command. I gave the input file name as  /dev/rmt0 (default)
and blocksize as 4096. Then its saying "Reading Elevation Tape...     
ERROR: error reading tape". The current window settings I am using are differentfrom the data settings I am trying to read. Do I need to adjust the window      settings before reading the data?   
Could some one help me out?

Thankyou  

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