FreeNAS (11.2-U7) how to repair degraded boot pool that is on two USB sticks?

I have a Dell R230 which I've setup a few years ago with FreeNAS (installed version is 11.2-U7) The boot partition was installed on a mirrored pool on two 32GB USB sticks. - /dev/da9p2 - SanDisk Ultra Fit USB 3.1 Flash Drive - /dev/da10p2 - Samsung Bar Plus USB 3.1 Flash Drive A while ago I started to get a warning when booting the server: info Boot Pool Status Is DEGRADED: One or more devices has experienced an unrecoverable error. An attempt was made to correct the error. Applications are unaffected. The Boot Pool Status looked like this: I removed the da10 device (Samsung Bar Plus) since that one showed 0 bytes written to /read from and rebootet, getting the same Boot Pool Status, but with /dev/da10p2 now reading INVAL instead of ONLINE. I clicked the three dots right of /dev/da10p2 and selected 'detach', and now I'm seeing this BootPool Status: So, the 'mirror' is gone and only /dev/da9p2 is still visible. Clicking the three dots right of /dev/da9p2 only gives me a 'replace' option and clicking the three dots right of freenas-boot gives me an 'attach' option. I got a second 32GB SanDisk Ultra Fit (so the same I hope as the one that is in /dev/da9p2, diskinfo -v reports exact same media size and sector size, but different Disk descriptors) - but when I try to attach it to freenas-boot, I'm getting the following gpart error: [EFAULT] Command '('gpart', 'create', '-s', 'gpt', '/dev/da10')' returned non-zero exit status 1. if I try to manually execute gpart create -s gpt /dev/da10 I'm getting gpart: geom 'da10': File Exists How can I get back to a mirrored boot pool using the second SanDisk Ultra Fit?