Okay...
1) Closed calibre, ran as below, result was
Code:
Filesystem Size Used Avail Use%
Memory 419M 27M 392M 6%
Stick 0 0 0 0%
Card 3830G 7M 3822G 0%
(There is no Memory Stick in the machine.)
2) Disconnected Reader, ran prs500df, message "Unable to find a connected ebook reader."
3) Reconnected Reader, ran prs500df, got exactly the same result as in (1) above, recognizing the SD card.