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So in trying to create an MBR partition of my hard drive which I could then use to boot a solaris-derivative OS using rEFIt, I messed up. Rather than a hard drive with a primary GUID partition map scheme to boot OS X 10.7 from, and an auxiliary partition with an MBR scheme for non-Mac OS's, I now have a hard drive that strangely enough, is formatted with an MBR scheme. Additionally, the very helpful recovery HD is gone, and I don't think i'll be able to reinstall it (http://apple.stackexchange.com/questions/19145/how-can-i-create-or-recreate-a-lion-recovery-partition), without a normally formatted HD.

In Terminal:

diskutil list
    /dev/disk0
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     FDisk_partition_scheme                        *500.1 GB   disk0
   1:                  Apple_HFS Macintosh HD            500.1 GB   disk0s1

In Disk Utility:

Disk Description : APPLE HDD HTS547550A9E384 
Media Total Capacity : 500.11 GB (500,107,862,016 Bytes)
Connection Bus : SATA
Write Status : Read/Write
Connection Type : Internal
S.M.A.R.T. Status : Verified
Partition Map Scheme : Master Boot Record

Not surprisingly, this initially caused some problems in that neither Macintosh HD nor the Recovery HD were being seen by EFI, but I was able to erase the hard drive, and restored my OS via Time Machine.

This leaves me in a tenuous situation. Messing with partitions and such is always risky business, and if anyone can come up with a reliable and trusted method for returning my HD to its native GUID'ness, it would be much appreciated! As I have a Time Machine, I'd be totally ok with completely erasing my HD and reinstalling lion.

EDIT:

sudo fdisk /dev/disk0
Disk: /dev/disk0    geometry: 60801/255/63 [976773168 sectors]
Signature: 0xAA55
     Starting       Ending
 #: id  cyl  hd sec -  cyl  hd sec [     start -       size]
------------------------------------------------------------------------
 1: AF 1023 254  63 - 1023 254  63 [         1 -  976773167] HFS+        
 2: 00    0   0   0 -    0   0   0 [         0 -          0] unused      
 3: 00    0   0   0 -    0   0   0 [         0 -          0] unused      
 4: 00    0   0   0 -    0   0   0 [         0 -          0] unused
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Can you just restore from Time Machine again? In that case, wipe the disk in Disk Utility using the "Partition" tab, clicking "Options..." and selecting the GUID Partition Table format. If that's not an option, please add the output from the command sudo fdisk /dev/disk0 (or whatever number your disk is) to the question - depending on the partition layout, there may not be enough space for a GPT. It certainly looks like there won't be enough space for an EFI System Partition, which means you won't be able to do firmware updates on this Mac unless you start over. –  pmjordan Aug 2 '12 at 8:05
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1 Answer

up vote 1 down vote accepted

As your Partition starts on sector 1, there is no space for a GUID partition table, which requires 2 sectors between the MBR (sector 0) and the start of the first partition as a minimum. You could try to move the partition with a specialist tool (the tools that come with OSX only let you change the size of a partition, not the starting position).

However, as you're OK with re-installing the OS, I suggest you make sure everything is backed up via Time Machine, then boot from the OSX installation stick. Open up Disk Utility before starting the installation, select the whole drive and open the "partition" tab. Repartition it from scratch by selecting "1 partition" (or however many you want) from the "partition layout" dropdown. Before you apply the changes, click the "Options..." button and choose the GUID Partition Table scheme. Then apply the changes (obviously, this wipes the disk) and restore OSX to the newly created system partition. This will create the GPT, protective MBR and EFI System partition.

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This is how I ended up doing it. I booted from my time machine HD by holding alt/option during boot. While my HD wouldn't show up at boot, my time machine would. Booting from this volume, I was able to get to Disk Utilities, and format my internal HD back to GUID. Thanks for the help, –  Nathaniel Gentile Aug 3 '12 at 4:28
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