Looks like a bug in the 7300 disk driver

[Leslie Mikesell]

>>In the UNIX-PC, the RAW -- read that "character-device" -- interface
>>to the disk directly maps transfers into user RAM.  Therefore, since
>>transfers to/from the disk are in 512 byte blocks, ya can only read
>>in 512-byte multiples.

For even more fun try writing from a buffer in a shared memory
segment -- the machine locks up competely.  (I ran into this
while trying to write a high-speed multi-process dump eventually
intended for a 3B2 tape drive.  After crashing the 3B2 by putting
the tape in streaming mode and writing too large a block, I thought
I would work the bugs out on a 3B1. Oh, well..)

Les Mikesell