Get a USB hub (7 port is common), plug the USB drives in, then run a script that copies the iso to one drive after another. USB itself sucks enough that trying to do them in parallel is likely asking for trouble.

Not on Windows, maybe WSL? Popsicle from System76

I have done a similar thing in the past, but to flash firmware onto any device with a certain USB descriptor that gets plugged in. It was a mess of USB hubs and cables, but it worked.

What I did was write a udev rule that checks for the vendor and product id of a newly plugged in device and calls a script when there's a match. The script then performs the flashing and logs the output.

In your case:

1. dd the source USB to a file (make sure the partition you're dding is smalled than any target drive
2. Udev rule according to your needs (all the same product or different drives?)
3. Script that dds the file you created earlier back to the newly plugged in drive.

Edit. Did this on a rpi3

Since you asked for windows, etcher can do multiple drives as of v1.4.3

https://blog.balena.io/etcher-now-with-multi-write-and-compute-module-support/

If you use a raspberry pi for each USB device, you could use multicast to distribute the iso across the network once and have each pi write it to the USB drive connected locally.

I also had a quick look around and found this:

• https://www.everyusb.com/how-do-you-copy-data-to-multiple-flash-drives-at-once

One other idea I came across was to setup the devices as a raid array and write to the raid device.

Does your windows machine have WSL? I would just write a bash script that would DD the image to the drives. Keep in mind that you will be severely bandwidth limited by the USB interface, so it will take a long time. All of the ports on the controller share the same bandwidth. Ideally you would build a PC with lots of USB controller cards for that.