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I keep receiving this error:

Warning!! Unsupported GPT (GUID Partition Table) detected. Use GNU Parted

I want to go back to the normal MBR. I found some advice here and did:

parted /dev/sda
mklabel msdos
quit

But when I get to the mklabel option it spits out a warning that I will lose all data on /dev/sda. Is there a way to get the normal MBR back without formatting the disk?

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What is your reason for wanting to switch? What is the real problem you are trying to solve? –  jordanm Jan 13 '13 at 17:20
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2 Answers

up vote 7 down vote accepted

That link you posted looks like a very ugly hack type solution.

However, according to the man page, gdisk, which is used to convert MBR -> GPT, also has an option in the "recovery & transformation" menu to convert GPT -> MBR; the g key will:

Convert GPT into MBR and exit. This option converts as many partitions as possible into MBR form, destroys the GPT data structures, saves the new MBR, and exits. Use this option if you've tried GPT and find that MBR works better for you. Note that this function generates up to four primary MBR partitions or three primary partitions and as many logical partitions as can be generated. Each logical partition requires at least one unallocated block immediately before its first block.

I'd try that first.

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Easily yes, but it is still risky. –  jordanm Jan 13 '13 at 17:19
    
@jordanm: I haven't done this myself (which is why I admit "google implies"), but since gdisk apparently has an explicit means, why do you believe it is risky? The gdisk man page for the r->g option does not have any caveats at all... –  TAFKA 'goldilocks' Jan 13 '13 at 17:25
    
It'd improve this answer if you were to inline the required gdisk step into your answer. Especially with how short the step is. Never know when the link may go dead, leaving the answer useless. –  derobert Jan 8 at 17:57
    
@derobert : Yeah, the original was more of a comment. I've used stuff from the gdisk man page instead. –  TAFKA 'goldilocks' Jan 8 at 18:19
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Most of the previous respondents are correct when they say that you can usually do what you need with a GPT partition table layout, but since there are valid reasons for wanting to use MBR, I think I will just answer your question instead of assuming you are wrong for asking.

To get rid of the GPT, you need to remember that with this format, there is a partition table written to the end of the drive, where it will remain if you just try to delete it with fdisk or a non-GPT aware file system tool. What you did with parted did not address this partition table.

To erase GPT, you need to use something like gdisk. Enter gdisk as root at the prompt, and then tell gdisk what device you want to look at (i.e. /dev/sd??). Use gdisk to write a protective MBR to the disk just to make sure you have access to some MBR data structure. Then you can navigate to the expert options section (press ? at the different program prompts to see the options available to you at different times) and find the option that says "Zap (destroy) the GPT data structures and exit." The program will prompt for confirmation, then ask you if you want to preserve the MBR structure. Do preserve this.

After that, it should be as simple as rebooting your computer (because the kernel will still be using the old partition table and it needs a restart to update), and then firing up fdisk and deleting the remnant of the GPT partition. You will still see the same warning about using GNU Parted that you saw before, but you can ignore it this time, as it will go away when you delete the partition.

This will give you an MBR partition table system on a blank drive onto which you can reinstall an OS. If you are attempting to do this with existing partitions, it is quite a bit more complicated and in some cases not even possible, so I would recommend that you simply back up your data and do it this way.

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"I think I will just answer your question instead of assuming you are wrong for asking." -- that's just crazy enough to work! –  Michael Mrozek May 4 '13 at 4:02
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protected by Michael Mrozek Jan 8 at 18:17

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