Thanks to all who replied. The consensus amoung replies was to ensure that

I was using the correct device name (I am), and to bulk erase the tape (I'm

not but will be soon). Some suggested that this is the result of a problem

with the st driver. Seems that SunOS 4.1.x will not allow tapes written as

low density to be overwritten as high density, and Solaris 2.3 will not

allow the reverse.