I’ve done that process with some success and failure. The failure was taking a running system with single USB drive, and mirroring to an SATA drive. It did not boot properly after shutdown and removing the USB drive.
USB boot and then adding a second USB to form the mirror worked fine. Make sure to test boot on each drive!
Keeping a backup of the exact iso I was running, and the config backup has proven to be a very good practice and easy way to get back up and running after a USB failure. I think the last time I did this was about 30 minutes start to finish. Usb3 drives have not worked well for me, they all live a short life generally from heat. Same for usb drives connected internally, even if they have good fans on them.