Can't Detach Mirrrored Boot Pool Device

I’m trying to detach a USB backup disk on a mirrored boot-pool in SCALE but the USB device appears under Disks as ‘sdc’ with one serial number and in the boot pool as ‘sdc2’ with a different serial number.

To top it off, regardless of which serial number I use, when I run 'sudo zpool detach freenas-boot [SERIAL NUMBER]’, I’m told ‘cannot detach [SERIAL NUMBER]: no such device in pool.’

How can I detach this drive?