reading DEM data

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

Subbarao Vedula