HP MicroServer Gen8: Boot TrueNAS from an SSD on the ODD Port using a GRUB USB Bootloader

The HP ProLiant MicroServer Gen8 is still a very useful small home server. It works well for TrueNAS, homelab setups, backup targets, or compact storage systems.

A common issue appears when you want to move away from booting TrueNAS from a USB flash drive and instead use a proper small SSD.

The MicroServer Gen8 has an internal SATA connector at the top of the chassis. This port was originally intended for an optical DVD drive. It is commonly called the ODD port and is mechanically very convenient for installing a small 2.5-inch SATA SSD.

The problem: with the four front drive bays populated, the server often does not boot directly from the SSD connected to the ODD port. Without the front drives installed, the SSD boots fine. As soon as the data drives are inserted again, the Gen8 usually tries to boot from Hard Drive C:, meaning one of the front bay drives.

The solution: use a small USB stick only as a GRUB bootloader. The actual TrueNAS installation remains on the SSD connected to the ODD port. The USB stick is barely written to and is therefore much less critical than using a USB stick as the full TrueNAS boot device.

Goal of the Setup

The final boot process looks like this:

HP MicroServer Gen8 BIOS
        ↓
internal USB stick with GRUB
        ↓
GRUB chainloads the SSD on the ODD port
        ↓
TrueNAS starts from the SSD
        ↓
data drives remain in the four front bays

The USB stick does not contain TrueNAS itself. It only contains GRUB and a small configuration file that starts the bootloader on the SSD.

Requirements

You need:

HP MicroServer Gen8
TrueNAS installed on an SSD connected to the ODD SATA port
a small USB stick or internal SD card
a Linux system to create the GRUB USB stick
GRUB package for BIOS/Legacy boot

In my setup, the SSD on the ODD port appeared to GRUB as:

hd5

This can vary depending on drive order and BIOS detection. During the first test, it is useful to try several options or check the available drives from the GRUB console using ls.

Important: SATA Mode in BIOS

For TrueNAS, the onboard SATA controller should run in AHCI mode.

F9 → System Options → SATA Controller Options

AHCI should be enabled. Using the B120i RAID mode is not recommended for TrueNAS. TrueNAS works best when it can access the drives directly.

Prepare the USB Stick

First, identify the correct USB stick on your Linux system:

lsblk -o NAME,SIZE,MODEL,TYPE,MOUNTPOINT

Example output:

sda    500G Samsung_SSD disk
sdb      8G USB_Stick   disk
└─sdb1

In this example, the USB stick is:

/dev/sdb

Warning: The following commands will completely erase the selected device. Make sure you have selected the correct USB stick.

Partition and Format the USB Stick

The USB stick is prepared with a classic MBR partition table and a small Linux partition.

sudo umount /dev/sdb1 2>/dev/null

sudo wipefs -a /dev/sdb

sudo parted -s /dev/sdb mklabel msdos
sudo parted -s /dev/sdb mkpart primary ext2 1MiB 100%
sudo parted -s /dev/sdb set 1 boot on

sudo mkfs.ext2 -F -L GRUBGEN8 /dev/sdb1

Now mount the partition:

sudo mkdir -p /tmp/usb
sudo mount /dev/sdb1 /tmp/usb
sudo mkdir -p /tmp/usb/boot

Install GRUB in Legacy BIOS Mode

The MicroServer Gen8 boots in classic BIOS/Legacy mode in this setup. Therefore GRUB must explicitly be installed as i386-pc.

On Debian, Ubuntu, or Proxmox, install the required package if needed:

sudo apt update
sudo apt install grub-pc-bin

Then install GRUB to the USB stick:

sudo grub-install \
  --target=i386-pc \
  --boot-directory=/tmp/usb/boot \
  --recheck \
  /dev/sdb

Important:

Correct: /dev/sdb
Wrong:   /dev/sdb1

GRUB must be installed to the whole USB stick, not only to the partition.

Special Case: Proxmox

If you create the stick on a Proxmox system, grub-install may be blocked by proxmox-boot-tool. You may see a message saying that grub-install is disabled.

In that case, use grub-install.real directly:

sudo /usr/sbin/grub-install.real \
  --target=i386-pc \
  --boot-directory=/tmp/usb/boot \
  --recheck \
  /dev/sdb

If you see this error:

cannot find EFI directory

then the following parameter is usually missing:

--target=i386-pc

Without this parameter, GRUB tries to install an EFI bootloader on a UEFI-booted Linux system. For the MicroServer Gen8 USB chainloader, we need classic BIOS GRUB instead.

Create the GRUB Configuration

Now create the actual GRUB boot configuration:

sudo nano /tmp/usb/boot/grub/grub.cfg

In my setup, the SSD on the ODD port was detected by GRUB as hd5. Therefore the final configuration looks like this:

set timeout=3
set default=0

menuentry "TrueNAS SSD on ODD Port - hd5" {
    insmod chain
    set root=(hd5)
    chainloader +1
    boot
}

With this configuration, GRUB waits three seconds and then automatically boots the SSD on the ODD port.

Alternative Test Configuration

If you do not yet know the correct drive number, you can create several menu entries for testing:

set timeout=10
set default=0

menuentry "TrueNAS SSD on ODD Port - hd4" {
    insmod chain
    set root=(hd4)
    chainloader +1
    boot
}

menuentry "TrueNAS SSD on ODD Port - hd5" {
    insmod chain
    set root=(hd5)
    chainloader +1
    boot
}

menuentry "TrueNAS SSD on ODD Port - hd3" {
    insmod chain
    set root=(hd3)
    chainloader +1
    boot
}

menuentry "GRUB Shell" {
    echo "Press 'c' to open the GRUB console."
    echo "Use 'ls' to list detected drives."
    sleep 5
}

During the first boot, test which entry works. Once the correct entry is known, reduce the grub.cfg to that single entry and set the timeout to three seconds.

Manually Check GRUB Drives

If none of the prepared entries work, press c in the GRUB menu to open the GRUB console.

Then run:

ls

Example output:

(hd0) (hd1) (hd2) (hd3) (hd4) (hd5)

You can then test manually:

set root=(hd5)
chainloader +1
boot

If that does not work, try:

set root=(hd4)
chainloader +1
boot

Or:

set root=(hd3)
chainloader +1
boot

As soon as TrueNAS starts, you have found the correct drive number.

Unmount the USB Stick Cleanly

After writing the configuration:

sync
sudo umount /tmp/usb

The USB stick can now be inserted into the internal USB port of the MicroServer Gen8.

Set the BIOS Boot Order

In the MicroServer Gen8 BIOS:

F9 → System Options → Boot Options

The internal USB stick should be placed before the hard drives:

1. USB DriveKey / Internal USB
2. Hard Drive C:
3. Network Boot

The server will then boot GRUB from the USB stick first. GRUB will then start the SSD on the ODD port.

Why Not Boot TrueNAS Directly from a USB Stick?

TrueNAS can technically boot from USB sticks, but it is not recommended for long-term reliability. USB sticks are a common source of boot-pool problems, especially with write activity, updates, and ZFS errors.

A better layout is:

TrueNAS system: SSD
USB stick: only GRUB bootloader
Data: separate ZFS data drives

In this setup, the USB stick has almost no write activity. Even if it fails at some point, it can easily be recreated. The actual TrueNAS installation remains safely on the SSD.

Error Example: GRUB Rescue / normal.mod not found

When an old TrueNAS USB boot stick fails, you may see an error like this:

GRUB loading..
Welcome to GRUB!

error: file 'normal.mod' not found.
grub rescue>

This usually means that GRUB can no longer find its normal module path or that the boot medium is damaged. If the TrueNAS boot pool also shows checksum errors, it is not worth trying to keep using that USB stick in production.

In that case, reinstalling TrueNAS on an SSD and restoring/importing the TrueNAS configuration is the cleaner solution.

Final Result

After successful setup, the boot process looks like this:

MicroServer Gen8 starts
USB-GRUB appears briefly for 3 seconds
GRUB starts hd5
TrueNAS boots from the SSD on the ODD port
Data drives remain untouched in the front bays

This keeps the MicroServer Gen8 fully usable:

4 front bays for data drives
ODD port for the TrueNAS boot SSD
internal USB port only for GRUB
AHCI mode for clean TrueNAS operation

Conclusion

This setup is pragmatic, robust, and much better than running the TrueNAS boot pool from an aging USB flash drive. The small GRUB USB stick only acts as a pointer, while the actual system boots properly from the SSD.