John Wilson confirmed that his program was designed to work with one floppy and an HDD. He says strange things happen if one tries to use two floppy drives instead... just as I found ;) I removed the second floppy drive, dug out an old 540 MB hard drive (with Win 95 on it) and hooked it up to the PC. Started Win95, then "Restart the computer in MS-DOS mode", copied PUTR to the C: drive and started it. PUTR now works perfectly, transferring files in both directions to an the emulated RX33 (3.5" floppy). The PDP-11 can read and write those disks on its generic 3.5" floppy "RX33", too. :) Now I just have to figure out the PC partitions/hard drives to make using PUTR as simple as possible.