Options to modify legacy boot partition for UEFI boot?

Froozle

Hi there! I've posted a question relating to this topic recently on the GhostBSD subreddit, but have yet to receive any response.
I'm relatively new to BSD, but I do have some reasonable amateur experience with Linux.

I've been working with a recent installation of GhostBSD on an old laptop after I had and tried and liked it on another PC.
This laptop system does not yet contain any critical data, but I have spent some time configuring, and had almost finished setting things up just the way I wanted.
The last thing I wanted to address (probably should have checked this earlier) was to try and change the boot mode from Legacy BIOS to UEFI.
Initially I had done the installation on this laptop using a liveUSB booted in Legacy BIOS mode, and I had opted not to install grub or rEFInd, as I wasn't planning to dual boot on this system.
What I did not realise was that those choices had left me with a very small _500k default "boot" partition.

Before I realised the size issue, I had tried and failed to simply copy the UEFI bootloader file into the existing boot partition, after reformatting the partition to FAT12.
Obviously this was a mistake. The laptop is not bootable in this state, and I am faced with the following three options I'd like to ask about here:

(1.) Would it be possible to manipulate the existing partitions from a GhostBSD live USB session to create a larger boot partition? E.g. something like this:

A. Reduce the main partition by half and duplicate it into the second half B. Delete the first half of the main partition C. Increase the size of the boot partition D. Recreate the main partition in the remaining space, copy the data back to the main part E. Delete the duplicate partition F. Resize the main partition to fill the space G. Generate the required UEFI boot file(s) on the newly resized boot partition

(2.) If the above plan is not feasible, how would I go about restoring the existing legacy boot partition to just go back to booting the system in Legacy BIOS mode as before?
I did try just once to fix it with the following command, but it didn't work. The system would not boot.
'gpart bootcode -b /boot/pmbr -p /boot/gptboot -i 1 ada0'
It was pretty late at the time so I haven't tried anything else yet.

(3.) The last resort would be to do a full reinstall, but I feel it would be a missed learning opportunity to just give up and take this route.

DHOC_TAZH

Here's a lengthy way of doing so. It could work. You'll need a live Linux USB stick. A dedicated stick or running a live Linux ISO image from Ventoy should work.

Copy and paste this, the page works. Not sure why the forum isn't previewing the link correctly.

gist.github.com/cjyar/cd5ea76a8692516767672ffc2883df92

Froozle

DHOC_TAZH many thanks for the reference, I'll be sure to check that out.

Meanwhile, by some cunning move I was able to pull off a recovery of the existing legacy boot without needing to reinstall! After wrangling with Brave browser's AI chat bot for a fair while, I was eventually able to figure out the required steps for recovery from a liveUSB, and why the initial steps didn't work.

For context, this was a default legacy boot installation of GhostBSD on a blank SSD with GPT partition table, done with a recent GhostBSD iso on a Ventoy USB stick.

For some reason, "gpart show" from the liveUSB was always reporting corrupted partitions, but would happily cooperate after using "gpart recover ada0".

Humorously, one of Brave's first suggestions was this... I opted not to accept this proposal:
gpart destroy -F ada0

On a more sensible side, the proposed fix included the following:

gpart show
gpart recover ada0
gpart show ada0
gpart delete -i 1 ada0
gpart add -t freebsd-boot -s 512k -i 1 ada0
gpart bootcode -b /boot/pmbr -p /boot/gptboot -i 1 ada0

then reboot

This didn't work however, on rebooting I saw the following errors:
"gptboot: backup gpt header checksum mismatch
gptboot: no ufs partition was found"

There were 2 reasons this did not work:
Firstly, the existing (legacy) freebsd-boot size allocation was only 256k, so it was adding a new ada0p1 partition after the main partition. Secondly, the main partition was zfs format, not ufs.

This fix did work for me:

gpart show
gpart recover ada0
gpart show ada0
gpart delete -i 1 ada0
gpart add -t freebsd-boot -s 256k -i 1 ada0
gpart bootcode -b /boot/pmbr -p /boot/gptzfsboot -i 1 ada0

then reboot

At this point I think I will just leave this laptop in legacy boot mode until I have more time to do a full reinstall.

screenshot-at-2026-05-14-00-33-08.png

me12345

Wow that is impressive. I would have never tried this. Backup copy and data with a reinstall is faster and much easier. Ah but you are right you would learn all fun stuff with the building requirements.