Bug in PUTR?

I'm having trouble copying files from my PDP-11 (RT-11 format) into an old Windows box using the last version of PUTR. It appears that WinXP does strange things with the hardware (3.5" 1.44 MB drives aren't actually RX33's although my RQDX3 controller believes they are). So I made an MS-DOS boot disk and run PUTR directly on MS-DOS (instead of the WinXP DOS window). Unfortunately MS-DOS 6.22 can't recognize my hard drive since it's NTFS-formatted, so it all has to be done in floppies. Both WinXP and MS-DOS know that A: and B: are two separate drives. Likewise the BIOS settings. I can copy files in DOS and Windows back and forth between the two drives. And I can MOUNT B: as a logical device DU0: (or without a logical device name, as B: /RX33 /RT11), and read its directory. But when I try to copy a file from A: to DU0:, the B: drive light flashes briefly, and then PUTR tries to write over the A: drive (blocked by the write- protect tab once I wised up)! So how on earth can the BIOS, MS-DOS and WinXP all know that A: and B: are two separate drives, but PUTR tries to write to A: even though the command is to write B: ?? I also tried switching the PUTR disk into B: and the RT-11 formatted disk to drive A:. Same problem (tries to write over the source disk which is now B: even though the output filespec is clearly A). I had a look at the code but nothing's leaping out at me. Although it's been many years since I wrote any 8086 code...