Trial with Diff7. Oddly, with Diff7, first try to load /boot/loader.efi fails and loaded from the next drive, although actually /boot/loader.efi exists in first drive (specified one, boot1.efi is read from). "probe: * supported (preferred)" line wasn't shown at all. To ease comparing with previous test, test No. is matched as before. So you would feel odd reading this file only. For internal disks, ZFS only have loader.efi. USB memstick has no ZFS and has loader.efi in UFS. ======================================================================== Output of gpart show with USB memstick attached. Note) diskid/DISK-S21MNXAG821947P is ada1. => 34 2000409197 ada0 GPT (954G) 34 2014 - free - (1.0M) 2048 409600 1 efi (200M) 411648 1024 2 freebsd-boot (512K) 412672 1024 - free - (512K) 413696 20971520 3 freebsd-swap (10G) 21385216 10485760 4 freebsd-ufs (5.0G) 31870976 1968537600 5 freebsd-zfs (939G) 2000408576 655 - free - (328K) => 34 488397101 ada1 GPT (233G) 34 2014 - free - (1.0M) 2048 204800 1 efi (100M) 206848 1024 2 freebsd-boot (512K) 207872 1024 - free - (512K) 208896 20971520 3 freebsd-swap (10G) 21180416 16777216 4 freebsd-ufs (8.0G) 37957632 450439168 5 freebsd-zfs (215G) 488396800 335 - free - (168K) => 34 488397101 diskid/DISK-S21MNXAG821947P GPT (233G) 34 2014 - free - (1.0M) 2048 204800 1 efi (100M) 206848 1024 2 freebsd-boot (512K) 207872 1024 - free - (512K) 208896 20971520 3 freebsd-swap (10G) 21180416 16777216 4 freebsd-ufs (8.0G) 37957632 450439168 5 freebsd-zfs (215G) 488396800 335 - free - (168K) => 3 1617794 da0 GPT (1.9G) [CORRUPT] 3 1600 1 efi (800K) 1603 34 2 freebsd-boot (17K) 1637 1614112 3 freebsd-ufs (788M) 1615749 2048 4 freebsd-swap (1.0M) ======================================================================== 2) Boot from ada0 without USB memstick attached (2 drives). ada0 should boot, but ada1 boots. >> FreeBSD EFI boot block Loader path: /boot/loader.efi Initializing modules: ZFS UFS Probing 12 block devices... boot1 imagepath: pciroot(0x0):pci(0x1f,0x02):sata(0x0,0x0,0x0):hd(1) probing: pciroot(0x0):pci(0x1f,0x02):sata(0x0,0x0,0x0) probe: . not supported probing: pciroot(0x0):pci(0x1f,0x02):sata(0x1,0x0,0x0) probe: . not supported probing: pciroot(0x0):pci(0x1f,0x02):sata(0x1,0x0,0x0):hd(1) probe: . not supported probing: pciroot(0x0):pci(0x1f,0x02):sata(0x1,0x0,0x0):hd(2) probe: . not supported probing: pciroot(0x0):pci(0x1f,0x02):sata(0x1,0x0,0x0):hd(3) probe: . not supported probing: pciroot(0x0):pci(0x1f,0x02):sata(0x1,0x0,0x0):hd(4) probe: + supported probing: pciroot(0x0):pci(0x1f,0x02):sata(0x1,0x0,0x0):hd(5) probe: + supported probing: pciroot(0x0):pci(0x1f,0x02):sata(0x0,0x0,0x0):hd(1) probe: . not supported probing: pciroot(0x0):pci(0x1f,0x02):sata(0x0,0x0,0x0):hd(2) probe: . not supported probing: pciroot(0x0):pci(0x1f,0x02):sata(0x0,0x0,0x0):hd(3) probe: . not supported probing: pciroot(0x0):pci(0x1f,0x02):sata(0x0,0x0,0x0):hd(4) probe: + supported probing: pciroot(0x0):pci(0x1f,0x02):sata(0x0,0x0,0x0):hd(5) probe: + supported done ZFS found the following pool: zsysS02 zsysS01 UFS found 2 partitions Failed to load '/boot/loader.efi' load: '/boot/loader.efi' spa: 'zsysS02', devpath: pciroot(0x0):pci(0x1f,0x02):sata(0x1,0x0,0x0):hd(5) load: '/boot.config' spa: 'zsysS02', devpath: pciroot(0x0):pci(0x1f,0x02):sata(0x1,0x0,0x0):hd(5) Failed to find '/boot.config' on pool 'zsysS02' (2) load: '/boot/config' spa: 'zsysS02', devpath: pciroot(0x0):pci(0x1f,0x02):sata(0x1,0x0,0x0):hd(5) Failed to find '/boot/config' on pool 'zsysS02' (2) Starting '/boot/loader.efi' in 5 seconds...... ======================================================================== 3) Boot from ada1 without USB memstick attached (2 drives). ada1 should boot, but ada0 boots. >> FreeBSD EFI boot block Loader path: /boot/loader.efi Initializing modules: ZFS UFS Probing 12 block devices... boot1 imagepath: pciroot(0x0):pci(0x1f,0x02):sata(0x1,0x0,0x0):hd(1) probing: pciroot(0x0):pci(0x1f,0x02):sata(0x0,0x0,0x0) probe: . not supported probing: pciroot(0x0):pci(0x1f,0x02):sata(0x1,0x0,0x0) probe: . not supported probing: pciroot(0x0):pci(0x1f,0x02):sata(0x0,0x0,0x0):hd(1) probe: . not supported probing: pciroot(0x0):pci(0x1f,0x02):sata(0x0,0x0,0x0):hd(2) probe: . not supported probing: pciroot(0x0):pci(0x1f,0x02):sata(0x0,0x0,0x0):hd(3) probe: . not supported probing: pciroot(0x0):pci(0x1f,0x02):sata(0x0,0x0,0x0):hd(4) probe: + supported probing: pciroot(0x0):pci(0x1f,0x02):sata(0x0,0x0,0x0):hd(5) probe: + supported probing: pciroot(0x0):pci(0x1f,0x02):sata(0x1,0x0,0x0):hd(1) probe: . not supported probing: pciroot(0x0):pci(0x1f,0x02):sata(0x1,0x0,0x0):hd(2) probe: . not supported probing: pciroot(0x0):pci(0x1f,0x02):sata(0x1,0x0,0x0):hd(3) probe: . not supported probing: pciroot(0x0):pci(0x1f,0x02):sata(0x1,0x0,0x0):hd(4) probe: + supported probing: pciroot(0x0):pci(0x1f,0x02):sata(0x1,0x0,0x0):hd(5) probe: + supported done ZFS found the following pool: zsysS01 zsysS02 UFS found 2 partitions Failed to load '/boot/loader.efi' load: '/boot/loader.efi' spa: 'zsysS01', devpath: pciroot(0x0):pci(0x1f,0x02):sata(0x0,0x0,0x0):hd(5) load: '/boot.config' spa: 'zsysS01', devpath: pciroot(0x0):pci(0x1f,0x02):sata(0x0,0x0,0x0):hd(5) Failed to find '/boot.config' on pool 'zsysS01' (2) load: '/boot/config' spa: 'zsysS01', devpath: pciroot(0x0):pci(0x1f,0x02):sata(0x0,0x0,0x0):hd(5) Failed to find '/boot/config' on pool 'zsysS01' (2) Starting '/boot/loader.efi' in 5 seconds....... ======================================================================== 5) Boot from da0 [USB memstick] (3 drives). da0 should boot, but ada0 boots. >> FreeBSD EFI boot block Loader path: /boot/loader.efi Initializing modules: ZFS UFS Probing 17 block devices... boot1 imagepath: pciroot(0x0):pci(0x1d,0x00):usb(0x00,0x00)usb(0x01,0x00):hd(1) probing: pciroot(0x0):pci(0x1f,0x02):sata(0x0,0x0,0x0) probe: . not supported probing: pciroot(0x0):pci(0x1f,0x02):sata(0x1,0x0,0x0) probe: . not supported probing: pciroot(0x0):pci(0x1f,0x02):sata(0x0,0x0,0x0):hd(1) probe: . not supported probing: pciroot(0x0):pci(0x1f,0x02):sata(0x0,0x0,0x0):hd(2) probe: . not supported probing: pciroot(0x0):pci(0x1f,0x02):sata(0x0,0x0,0x0):hd(3) probe: . not supported probing: pciroot(0x0):pci(0x1f,0x02):sata(0x0,0x0,0x0):hd(4) probe: + supported probing: pciroot(0x0):pci(0x1f,0x02):sata(0x0,0x0,0x0):hd(5) probe: + supported probing: pciroot(0x0):pci(0x1f,0x02):sata(0x1,0x0,0x0):hd(1) probe: . not supported probing: pciroot(0x0):pci(0x1f,0x02):sata(0x1,0x0,0x0):hd(2) probe: . not supported probing: pciroot(0x0):pci(0x1f,0x02):sata(0x1,0x0,0x0):hd(3) probe: . not supported probing: pciroot(0x0):pci(0x1f,0x02):sata(0x1,0x0,0x0):hd(4) probe: + supported probing: pciroot(0x0):pci(0x1f,0x02):sata(0x1,0x0,0x0):hd(5) probe: + supported probing: pciroot(0x0):pci(0x1d,0x00):usb(0x00,0x00)usb(0x01,0x00) probe: . not supported probing: pciroot(0x0):pci(0x1d,0x00):usb(0x00,0x00)usb(0x01,0x00):hd(1) probe: . not supported probing: pciroot(0x0):pci(0x1d,0x00):usb(0x00,0x00)usb(0x01,0x00):hd(2) vdev_read: failed dev: 0xdca13d30, id: 1, lba: 32, size: 114688, status: 2 dskread: failed dev: 0xdca13d30, id: 1, lba: 128, size: 8192, status: 2 probe: . not supported probing: pciroot(0x0):pci(0x1d,0x00):usb(0x00,0x00)usb(0x01,0x00):hd(3) probe: + supported probing: pciroot(0x0):pci(0x1d,0x00):usb(0x00,0x00)usb(0x01,0x00):hd(4) probe: . not supported done ZFS found the following pool: zsysS01 zsysS02 UFS found 3 partitions Failed to load '/boot/loader.efi' load: '/boot/loader.efi' spa: 'zsysS01', devpath: pciroot(0x0):pci(0x1f,0x02):sata(0x0,0x0,0x0):hd(5) load: '/boot.config' spa: 'zsysS01', devpath: pciroot(0x0):pci(0x1f,0x02):sata(0x0,0x0,0x0):hd(5) Failed to find '/boot.config' on pool 'zsysS01' (2) load: '/boot/config' spa: 'zsysS01', devpath: pciroot(0x0):pci(0x1f,0x02):sata(0x0,0x0,0x0):hd(5) Failed to find '/boot/config' on pool 'zsysS01' (2) Starting '/boot/loader.efi' in 5 seconds.......