I just finished the huge download of Mac OS X Lion, started the installer, and now I have a problem. The installer says that I must have the GUID-Partitions Table Scheme.

The problem is that when I go into the disk utility, I can't change the format anymore, because I have Master Boot Record as type.

Can anyone help me, please?

  • 1
    Which OS are you upgrading from? Commented Jul 20, 2011 at 17:48
  • I have heard of tools to change things on the fly but can't recommend them. Do you have a spare HD to hold the contents of the drive while it is erased and changed to the correct format? It's normally quite a trick to get Snow Leopard installed on MBR drives. Well done!
    – bmike
    Commented Jul 20, 2011 at 17:53
  • Can one upgrade from anything other than Snow Leopard?
    – lkraav
    Commented Jul 20, 2011 at 17:56
  • @lkraav: No. Upgrading to Lion requires Snow Leopard. That is why I asked my question in the comments. Commented Jul 21, 2011 at 0:19

2 Answers 2


Ok thanks for your help, but I solved the problem on my way, here's what I've done:

  1. Backup everything important to an external Harddrive
  2. Inserted the Mac Installation Disc and ruined there the disc utility app.
  3. I removed everything from my HD and it was GUID-Partioned.
  4. Now I just installed Mac Os X 10.6.2 and updated to 10.6.8
  5. Now I could run the installer of Mac Os X Lion without any problems and as a benefit of the whole thing, I started with a brand new Mac Os X Installation, without any trash in the file system.

search: http://www.google.ee/search?q=convert+mbr+to+guid&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a

http://www.insanelymac.com/forum/lofiversion/index.php/t183105.html claims that http://www.coriolis-systems.com/iPartition.php is able to do MBR -> GPT conversion.

Then there's supposedly some not-for-faint-of-heart options mentioned in that thread, which IMHO judging from the tone of the original question, are not safe enough to use. At least make a whole disk backup first.

  • I have used iPartition a few times to change partition schemes without destroying data partitions. Commented Mar 30, 2013 at 15:20

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