Anyone know what the correct (or perhaps mildly optimal) blocking
factor for tar and a ZIP disk may be?  I'm trying to do a full backup
of my system onto a handful of zips and I'm using (among other flags),
the --multi-volume and --tape-length=98304 flags to tar.  It mostly seems
to be running ok, but just as it finishes with 1 zip, It says:

sd2(sbc0:6:0): illegal request, data 00 00 00 00 21 00 00 00 00 00 0e 2d 01 06 a8 00

(same message 14 more times)

then it asks for the next volume.  The exact same thing happens at the
end of that.  

This is on a mac68k 1.3 system.  

Could this be caused by using too high a blocking factor?  Wront tape length?
I thought the zip had 96 cylinders, 2048 sectors/cyl, 512 bytes/sect = 
98304 * 1k bytes/ZIP


TIA,


Dan