Free software for faster msOS flashing. You can use SSD Flasher to reflash:
Download the SSD Flasher and flash the image with Etcher to a free USB like you would flash any other msOS image.
Download msOS image that you would like to flash to your rigs, unzip it, and copy/paste the .img file to the root folder of the freshly flashed USB.
You can find all msOS versions here.
Plug the USB to your rig and wait for USB Flasher to flash msOS image to your secondary SSD or HDD. You can repeat this step for as many rigs as you like.
Find more detailed instructions here.