Can't Mirror boot drive

[UMB8998]

I'm trying to attach a usb drive to mirror the boot drive, but I'm getting an error. My system is installed on a SSD partitioned at 28GB, and the USB is of 32GB.
When I try on a flash drive I get

Error: [EFAULT] newfs_msdos -F 16 /dev/da1p1 failed: newfs_msdos: /dev/da1p1: Operation not permitted

when I try to do it again i get

Error: [EFAULT] [EFAULT] Command gpart create -s gpt /dev/da1 failed (code 1): gpart: geom 'da1': File exists

I don't know what to do, please help