repost: Booting UEFI and GPT on p6-2155a with Foxconn 2ADA mboard

[PB442]

I am reposting as I resolved my problem.  If you are facing a similar issue, you may wish to also read the original post and replies which contained useful information,  The original question is below and I will next post where I ended up.

Hi.  I have a P6-2155a bought 12 years ago. It has the original processor - an Intel(R) Core(TM) i7-3770 CPU @ 3.40GHz  - and Foxconn 2ADA motherboard. It now also has 16.0 GB DDR3-1600 RAM, an upgraded power supply, an NVIDIA GeForce GTX 1050 Ti and a Crucial 4TB SSD.  It's running Windows 10 Home 64 bit. I gritted my teeth some years ago and upgraded the bios to 8.17 (JOS_817.rom issued in 2013), I think it was to run a previous NVIDIA card at the time.   

It is booting via legacy (ie BIOS) and if possible I would like to swap to UEFI and GPT to make full use of the space on the SSD. Using legacy BIOS means I am limited to 4 partitions and a maximum of 2.2 TB, so there's a fair bit of the SSD unusable - about 1.64 TB in practice.

Most importantly, I do not know if booting via UEFI rather than BIOS  would succeed given the old motherboard.  So that is my first question, can the motherboard work under UEFI?  Has anyone got a working PC using UEFI with a Foxconn 2ADA motherboard? Preferably in an HP computer that previously used AMI BIOS.

Thanks in advance for any assistance.  

[PB442 · Accepted solution]

 @Paul_Tikkanen and Prométhée

So a year and a bit later I came back to the problem with more time to sort it out, and succeeded.  The short version is that Recovery was not properly set up, missing both WinRE.wim and once that was copied in (extracted from the ISO using 7-zip) , needing to be enabled (using reagentc enable).  Once both were done, MBR2GPT /validate /disk:0 /AllowFullOS ran successfully instead of reporting an error (see previous thread). I then ran MBR2GPT /convert /disk:0 /AllowFullOS  successfully, converting the drive to GPT successfully.  I then edited the BIOS setup to disable legacy and enable secure boot.  Then restarted and hoped it worked.  It did. I then used AOMEI to expand the C: drive from 2TB (with just 92 GB free) to 3.7TB  (with 1.8TB free) which was the reason I started on this whole exercise last year - to be able to use all of a new 4TB SSD.  The reported error mentioned above when trying to convert to GPT using AOMEI was disk fragmentation, which didn't make sense to me on an SSD, and indeed wasn't the problem.

The longer version is I spent a bit of time chasing dead ends using diskpart, and boot configuration, but I'll spare you those. 

Posting this because it may be of use to someone else facing a similar problem.

So to be clear, I now have a 2011 HP desktop with a Foxconn 2ADA motherboard and an i7-3770 processor happily running Windows 10 and UEFI/GPT without having to do a clean (re)install of the OS. (NB I had previously successfully upgraded the AMI BIOS to a a version beginning with 8 back in 2015 - I think at the time to run a new NVIDIA graphics card.  This was successful and that version of the BIOS had the option for UEFI/secure boot - as far as I can remember the the original 2011 BIOS did not).

Thanks to all who tried to help early last year ( Prométhée and @Paul_Tikkanen ), and hope this is of use to someone.