How to convert a GPT disk with Win10 to MBR

[Andi B.]

I don't want to loose the Win10 installation. How can I convert the disk to MBR so I can add some OS/2 partitions?

[Pete]

Hi Andi

This is probably what you are looking for

    https://docs.microsoft.com/en-us/windows-server/storage/disk-management/change-a-gpt-disk-into-an-mbr-disk


Have fun  :-)

Pete

[Andi B.]

The say -

You can change a disk from a GPT to an MBR partition style as long as the disk is empty and contains no volumes.

I don't want to delete the W10 installation.

DFSee does have an option to convert GPT to MBR. But does the installed W10 still work afterwards?

[Pete]

Hi Andi

I have never needed to try.

You could email Jan (DFSee) and ask  :-)


Regards

Pete

[Joseph M. Kempf]

Andi:

If you have an opportunity to pose your question to Jan please let us know what he tells you.

I took a quick look at DFSee 15.3.  DFSee will clone partitions but it is unclear to me whether DFSee will clone a partition from a GPT disk to a MBR disk, and if so whether the MBR clone will thereafter boot with AirBoot.

With Windows 7 reaching the final end of its service life I've been trying to setup a Thinkpad to dual boot to ArcaOS and Windows 10.  I am having a devil of a time with it.  If and when I can actually get Windows 8.1 or 10 to install it crush not only AirBoot but reformats my ArcaOS partition at the end of the drive.

[Remy]

The say -

You can change a disk from a GPT to an MBR partition style as long as the disk is empty and contains no volumes.

I don't want to delete the W10 installation.

DFSee does have an option to convert GPT to MBR. But does the installed W10 still work afterwards?

From a bad experience I had, adding MBR to a GPT disk makes GPT partition unreadable.
Remember that GPT is for very large disk while MBR is for smaller disk. I suppose your windows 10 is on a large  partition, may be full disk size and above 2TB !

[Doug Bissett]

I have converted Win 10, on a GPT disk, to Win 10, on a MBR disk, a few times, while testing ArcaOS. My method should work okay, as long as the physical disk is not more than 2 TB.

Briefly:

First BACK UP WINDOWS. Having said that, the only backup program, that I know of, that actually works, is Acronis. Acronis does come in a couple of limited capability, free versions. One works, if you have a Seagate, Samsung, or some other brand (all owned by Seagate). The disk can be in an external enclosure, and doesn't need to be used for the backup. The other is from Western Digital (don't know much about that one). Google for Seagate Discwizard.

Before continuing, you may want to experiment with an old disk, if you have one. Using the disc wizard can be a bit confusing.

Next, use a stand alone DFSEE boot, to wipe the front end of the disk. Then use an OS/2 installer (preferably ArcaOS) to create ALL of the desired partitions (including windows). Do NOT, ever, allow any other disk utility (except DFSEE, running under OS/2) to mess with the partitions.

Now you can restore windows, and proceed to install OS/2 along side of it.

[Andi B.]

In DFSee there is a menu entry for changing GPT to MBR. Honestly I've not even read the documentation until now. So it's a bit to early to bother Jan with this. Have to check what Jan has written in his docs before.

Dough thanks for the hint to Acronis. I've read similar messages from you in the past but didn't remember were. Thanks again for answering here. I did have bad experience with backup and restore Win7 with DFsee so I wanted to do it different this time with W10. So I'll try 'your' Acronis way.

One question, currently there are three partitions on this 250GB SSD. One of them seems to be the recovery partition. I think I can safely delete this cause guess it would recreate a GPT system anyway which I don't like. Or would it be better to back up and restore this one too? Is it important for W10 to have the two other partitions in front of it's system partition or does it work without the recovery partition in between?

[Olafur Gunnlaugsson]

I don't want to loose the Win10 installation. How can I convert the disk to MBR so I can add some OS/2 partitions?

This may not be of any help to your use case (laptops with only one drive bay etc), but my preferred way of doing this is to keep it as simple as possible by buying one of those 20 euro 120 gb SSDs from China on ebay et.al. and using this as a MBR drive while keeping the windows drive as GUID. This gives me the added bonus of being able to skip using boot managers unless I want to have multiple OS/2 installs on the MBR drive, as I simply use the F10 (F11/F12 or whatever) boot selection as an alternative, if you choose EFI boot the UEFI bootloader only initially sees the GUID drive even though Windows will see that drive after initial boot, while if you choose the BIOS emulation boot the GUID drive gets ignored.

Similarly during install I simply boot the CD drive in either EFI or BIOS mode using the Fxx key depending on which OS I want to install, etc.

[Joseph M. Kempf]

Thanks Doug et al:

I've used Acronis in the past.  It works well.  I'll try setting up a dual boot Windows 10 /ArcaOS laptop in next few days using Acronis and DFSee.

[Doug Bissett]

One question, currently there are three partitions on this 250GB SSD. One of them seems to be the recovery partition. I think I can safely delete this cause guess it would recreate a GPT system anyway which I don't like. Or would it be better to back up and restore this one too? Is it important for W10 to have the two other partitions in front of it's system partition or does it work without the recovery partition in between?

Briefly:

Win 10  likes to have a boot partition, of about 100 MB (I think), which has no windows drive letter, followed by the C: drive. There may also be a "recovery" partition (usually at the end of the disk), which contains the original machine code load. What is in there, or how it works, seems to change from manufacturer to manufacturer, and it may only be bootable when using GPT.  It would likely be a good idea to image the whole disk, with DFSEE, so you can get back to what you started with BEFORE you start to mess with it.  The 100 MB partition is not necessary, unless you want to use one of those disk encryption programs (they live in that partition, along with the windows loader). The windows loader is also on the C: drive, but probably needs to be enabled, to bypass the small partition loader. Acronis figures that out for you, if you only restore the C: drive, but not if you do have the drive encrypted.

I've used Acronis in the past.  It works well.  I'll try setting up a dual boot Windows 10 /ArcaOS laptop in next few days using Acronis and DFSee.

It is probably easier to use the tools in the ArcaOS installer to initialize the disk for LVM, and create ALL of the partitions with that (it can also wipe the disk). Never allow any other partition manager (including DFSEE, if it is not running under OS/2) to touch the partitioning, or it is likely to be messed up. One of the READMEs, in the root of the ArcaOS installer DVD has "official" instructions.