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Is there any tool that can reliably convert a disk from GPT to MBR in-place?
(i.e. without having to backup/restore any partitions.)

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This conversion can be done with my gdisk program, The actual GPT-to-MBR conversion isn't all that complex (to the end user), although there are some significant caveats, such as:

  • If there are more than four partitions, some of them will have to be logical partitions under MBR, and those have spacing requirements that may not be met in the original. Thus, it may not be possible to convert all the partitions without resizing some of them.
  • Some GPT data, such as partition names, will be lost.
  • If the disk is over 2TiB (assuming 512-byte sectors), partitions that extend beyond the 2TiB mark won't be converted. MBR has a 2TiB limit on partition sizes and start points, which is one of the main reason it's being put out to pasture; it's just not useful on larger disks today.
  • If the disk is a boot disk, you may need to re-install your boot loader. This will be a real pain with Windows, but is likely to be easier with Linux.

The gdisk documentation covers the process in detail. The bigger question, though, is: why? There are legitimate reasons to want to do this conversion, but there are also a lot of cases when such a conversion would be ill-conceived. Knowing why you want to do the conversion would enable me to better advice you about whether converting to MBR makes sense or if there's some better way to achieve your ultimate goal.

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+1 I appreciate the answer. The part that concerns me (and that you didn't mention) is what happens if my computer came with Windows 8 preinstalled -- how does the process change (assuming my computer supports legacy mode), and does that invalidate my license? The "why" is basically that I want to be able to boot non-UEFI-compatible OSes (e.g. old Linuxes, etc.), which the UEFI Windows boot manager doesn't do. –  Mehrdad Sep 11 '13 at 17:34
    
@Mehrdad That same C't magazine spend 20 pages on just those issues. The GPT->MBR conversion story was one of those articles. If you can read Dutch I can recommend it. It is very thorough and explains in great detail. Issue 2013-10. –  Tonny Sep 11 '13 at 17:45
    
@Rod Smith Great program, your gdisk ! –  Tonny Sep 11 '13 at 17:46
    
@Tonny: lol I can't read Dutch, is there an English version? –  Mehrdad Sep 11 '13 at 17:55
    
Mehrdad, see my fourth bullet point: "If the disk is a boot disk, you may need to re-install your boot loader. This will be a real pain with Windows, but is likely to be easier with Linux." If Windows is booting from the disk, it will no longer boot from the disk after such a conversion -- at least, not until you re-install a boot loader. –  Rod Smith Sep 11 '13 at 21:46
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It is possible with a Linux tool called "gdisk"...

But the process is very complicated, dangerous and a straight conversion is not always possible.
(It highly depends on the original partition layout. Not everything converts from GPT to MBR, because MBR is more limited in features.)
If it is your boot-disk then afterwards you will have to mess with BCDBOOT/BCDBOOT to repair the Windows boot-process.
(Or do complicated chroot stuff to get Linux booting again.)

Example: The Dutch computer magazine C't had an article on this last month. It took them 3 full pages just to describe the process. (I won't repeat that here.)

Unless you are an expert in these matters (since you had to ask you are NOT) it is probably better if you don't even attempt this.
I would say that the full backup/restore process is easier and less dangerous.
(You will have to make a full backup anyway just to be safe before you do something like this. Might as well use it for the restore then.)

P.S. If you have a (pre-installed) Windows 8 computer that came with GPT as standard to process is even more complicated and it is even possible it will invalidate your (pre-installed) Windows license.
(It is possible the manufacturer setup the UEFI BIOS/Secure boot such that it will ONLY work on GPT.)

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up vote 0 down vote accepted

MiniTool Partition Wizard can do this.
I just tested in both directions (MBR <-> GPT) and it did the job in ~10 seconds.

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