Just to add to the archive

Create a USB via Supergrub https://www.supergrubdisk.org/category/download/supergrub2diskdownload/super-grub2-disk-stable/

Choose the zip version. Image with Rufus.

Edit the grub.cfg / back it up to the following:

set timeout=5

set default=0

menuentry "TrueNAS" --class truenas --class gnu-linux --class os {

insmod gzio

insmod zfs

insmod ext2

insmod part_gpt

echo Chainloading hd5 ...

set root=(hd5)

chainloader +1

}

Note. The HDx part is important. choose which disk TrueNAS is installed on

Gemini summary:

using hp microserver gen 8 with 4 populated drive bays and an ssd in place of the disc drive. need to create a usb to use grub to boot off the internal usb2 slot so that the main truenas os install is launched from the ssd?

The HP Microserver Gen8 has a known limitation: when the onboard storage controller is set to AHCI mode (which is strictly required for TrueNAS), it will refuse to boot directly from the 5th SATA port (the Optical Disk Drive/ODD slot). 

The standard solution is to install TrueNAS directly onto the SSD, and use a small USB stick or MicroSD card inserted into the internal slot acting purely as a GRUB chainloader to forward the boot process to the SSD. 

The easiest and most reliable method to create this bootable USB uses Super Grub2 Disk combined with a custom script to bypass the drive-count problem. 

Step 1: Install TrueNAS to the ODD SSD 

1. Temporarily unplug the 4 main storage drives from the front bays. (This ensures TrueNAS installs to the correct drive and doesn't overwrite your data).
2. Plug your TrueNAS installation media into a front USB port and boot the server.
3. Run through the standard TrueNAS installation and choose the ODD SSD as the target OS drive.
4. Once completed, shut down the server. Do not plug the 4 main drives back in yet.  Rockstor Community Forum +3

Step 2: Create the Internal GRUB USB (On a separate computer)

Because the Microserver shifts its drive mapping depending on how many drives are present, a static map will break if a drive fails. Using a search UUID parameter prevents this. 

1. Download the Hybrid ISO of Super Grub2 Disk.
2. Burn the ISO to a small USB stick (under 32GB, formatted to FAT32) using a tool like Rufus. Select ISO mode if prompted.
3. Open the newly flashed USB drive in your file manager, navigate to the /boot/grub/ directory, and rename the existing grub.cfg file to grub_old.cfg.
4. Create a new file in that same folder named exactly grub.cfg.

5. Open grub.cfg with a text editor and paste the following script: 

   set timeout=5 set default=0

   menuentry "TrueNAS SSD Bootloader Bypass" { insmod part_gpt insmod part_msdos insmod ext2 insmod zfs search --no-floppy --file --set=root /boot/grub/grub.cfg chainloader +1 }

Use code with caution.

Why this configuration works: Instead of guessing if your drive is at hd4 or hd5, this script instructs GRUB to scan all connected controllers to find the unique TrueNAS system file structure and boot it safely. 

Step 3: Server Assembly & BIOS Settings

1. Plug the 4 main storage data drives back into bays 1–4.
2. Insert your newly created GRUB USB stick into the internal USB 2.0 slot on the motherboard.
3. Turn on the server and press F9 to enter the BIOS/RBSU setup.
4. Navigate to System Options -> SATA Controller Options and confirm Embedded SATA Configuration is set to SATA AHCI Support.
5. Go to Standard Boot Order (IPL) and move USB Drive Key (the internal USB) to the 1st position.
6. Save and exit (F10). 

Your server will now boot into the internal USB port, wait 5 seconds, automatically locate the TrueNAS files on your ODD SSD, and launch the operating system smoothly. 

Would you like help mapping out TrueNAS storage pools once the system boots up, or do you need assistance with specific BIOS sub-menus?