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!