[Charles]

>But there is some kind of bug that always appears at the same point, in the
>middle of the next 100K block after "6600K written".
>The data transfer stops (no more head motion/ready light flicker on the
>RL02), and the character that vtserver uses to indicate a write operation
>just repeats endlessly and rapidly until I kill it.

More information and a correction: I let it run on, and it is still reading 
and writing, but at a much slower and intermittent rate than the first 6.6 
MB.
The filler character is just a time marker of some kind, since I can still 
see the "r" indicating a read from the .dsk image, and the light on the RL02 
flickers after a few of those.
So it's slowed way down, but not stopped! Even more mysterious.

Anyway, this disk has 13800 blocks out of 20800 used. If RT-11 stores data 
(including the directory structure) starting from block 0, I may be able to 
kill the writes after 7 MB.
(Unfortunately I think I neglected to squeeze the image before sending it to 
the RL - and naturally the important TSX files are near the end - which 
means I have to wait for most of it).
If I have to do it again, I'll squeeze and then kill it after 7 MB and see 
what I got!