The HP Microserver Gen8 when configured to disable RAID on the front loading bays (something you should do if planning on using ZFS) is unable to boot from the 5th SATA port, which is usually reserved for the front optical drive.
There are several workarounds, but the most popular one is installing a separate bootloader on an USB drive or MicroSD card, and from there load your OS of choice.
Grab your USB drive, create a MBR partition table and a FAT32 partition, and mark it bootable.
I was migrating a system that had been running directly on another USB drive, which
obviously was now dying. I dded the whole thing to a separate mSATA drive I had
laying around, and then booted FreeBSD again from the barely alive USB to finish
configuring the new bootloader drive.
I did this to automatically grab the optimal configuration for the running system,
but if you are doing this from a separate OS, there are a few links with information
on how to set up a grub.cfg to do chainload, or boot FreeBSD.
I however do not like that they use fixed hdX paths which may change if a drive dies
and then render the system unbootable. The grub configuration that is autogenerated from
the running system uses uuids instead. I have not investigated how to do this from a
separate system, but must be possible.
In any case, here, here and here are the links.
From my system I installed grub-pcbsd, installed it to my new USB drive and generated
a new configuration file:
# pkg install grub-pcbsd msdosfs
# mkdir -p /mnt/usb
# mount -t msdosfs /dev/da0s1 /mnt/usb
# mkdir /mnt/usb/boot
# grub-install --no-floppy --boot-directory=/mnt/usb/boot /dev/da0
# grub-mkconfig -o /mnt/usb/boot/grub/grub.cfg
Then shut the system down, unplug the old USB, connected the SATA drive with FreeBSD if it wasn't yet connected, and you're done.
Procedure should be the same but you cannot use grub-mkconfig to automatically
generate grub.cfg for you.