Using following kit

Atari 800XL 256k, Laptop, 2x1050s, Dual sio2pc-usb

trying to write out ATR to real 5¼" floppy, using Prosystem

Prosystem successfully detects disk drive
and allows me to choose ATR to write to disk then, clicking START produces following error msg:
Drive 1 Aborted (could not Open serial port)

things i've tried
swapping disks drives for other 1050s and Indus, Rana & XF551
swapping XL for 1200XL (ult 1mb), 800XE (576k), and stock 800XL
swapping SIO cables
re-installing APE?Prosystem software
opening up molex connectors - all contacts are secured

one thing worth noting
ALL combinations of the above will boot a real disk and format and copy from within the real Atari platform,

any help appreciated

Old question but anyway here's my experience. "Can not open serial port" error sometimes happen to me too, usually re-scanning drives will solve the problem. if not, exit Prosystem, unplug the SIO2PC, wait a few seconds and re-plug it, start Prosystem and see if it works this time. I am not sure why that happens, perhaps because it is a VCP port rather than a real serial port. Also, wait until the port scanning is completed before selecting any menu options, if you are too fast (faster than the scan) sometimes Prosystem will not detect the drive properly.

Ray