I read a tech doc over at www.austin.ibm.com and they have info on how to
deal with scsi tapes, etc... it's a pretty decent document.

They mention most tape drives can be put in either fixed-block or
variable-block modes.... is this true, and if so, how do I know which
mode NetBSD is using?  Can it be changed?

I'm looking at the st.c right now but it is a bit confusing.

Their SMIT utility actually does a pretty good job of showing you information
about the hardware - you know, which tape device deals with which density
on the tape, etc... all the stuff that is a serious pain in the butt when
you work with multiple OSes..
(I'm not saying it is done right, just that it is done)

I keep having to look up i386/isa/fd.c to remember which floppy device
is which size/density :)