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FreeBSD boot fail after zpool upgrade
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If you upgrade your zpool because it has "unactivated features"

zpool status
zpool upgrade -a

and find you are now stuck in this position...

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BIOS drive C: is disk0
ZFS: unsupported feature: com.klarasystems:vdev_zaps_v2

Can't find /boot/zfsloader

Can't find /boot/loader

Can't find /boot/kernel/kernel

FreeBSD/x86 boot
Default: disk-1:/boot/kernel/kernel
boot:

Can't find /boot/kernel/kernel

FreeBSD/x86 boot
Default: disk-1:/boot/kernel/kernel
boot: _

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1. Boot FreeBSD into single user mode from a USB stick
2. gpart show
3. gpart bootcode -b /boot/pmbr -p /boot/gptzfsboot -i 2 ada0
4. shutdown -r now