Thanks for this Rich. I'm running BigSur and noticed I couldn't move to /bin as you mentioned. I ran the mv command with /usr/local/bin and it appeared to work however I didn't yet have a bin folder in that location so it seems it moved the iperf3 tool there and renamed it to bin. Once I figured this out, I deleted it, created the bin directory and then ran the command again and it moved it as expected. Just thought I would mention this in case anyone else had the same experience.
These instructions below effectively replace the method described above. They specifically describe steps to replace the usb drive before purchasing. I'm new to Unraid so i'm not sure if the flash backup was previously possible. https://wiki.unraid.net/UnRAID_6/Changing_The_Flash_Device