What I've Tried
I've used the same cable to add books to the Reader and have never had any problems with it.
Nonetheless, I tried a new cable and a new port on my laptop. The Reader was detected quickly this time, so that may have had some impact. However, the copy to disk still failed with the same error.
I have 347 books on my Reader -- 202 on an SD card and 146 on the internal memory. I can copy the stuff off the SD card by removing it from the Reader and copying it directly. Is it possible to attach the Reader and make it appear as a USB drive (only) and copy the files off it that way?
I don't believe it's a port issue -- as I said, I've tried two different ones on the laptop without success. FWIW, I routinely copy 6 megapixel photos from my camera to the laptop without any difficulty.
I also tried copying to disk with the Reader plugged in with the AC adapter. That didn't make any difference, either.
Whatever is happening, it's leaving the Reader in a state that it thinks it's still copying. I've quit Calibre completely (no icon on the taskbar) and the Reader still has the "Don't unplug while copying" message.