FreeBSD mirror error: "geom: Not all disks connected"

Whilst copying data to my FreeBSD RAID1 mirror, one of the disks errored:

Jan 4 07:08:44    freenas     kernel: GEOM_MIRROR: Device RAID1: provider ad0 disconnected.

Jan 4 07:08:44    freenas     kernel: GEOM_MIRROR: Request failed (error=5). ad0[WRITE(offset=1632316556288, length=131072)]

Jan 4 07:08:44    freenas     kernel: ad0: FAILURE - WRITE_DMA48 status=51<READY,DSC,ERROR> error=10<NID_NOT_FOUND> LBA=3188118402

I also got this error:

 "geom: Not all disks connected."

ad0 had errored, leaving the RAID operating only with ad1.

The solution was posted here:

gmirror forget RAID1

gmirror status

      Name    Status  Components

            mirror/RAID1  COMPLETE  ad1

gmirror remove RAID1 /dev/ad0

gmirror: No such provider: ad0.

gmirror insert RAID1 /dev/ad0

gmirror status

      Name    Status  Components

            mirror/RAID1  DEGRADED  ad1  ad0 (0%)