Macos – Change partition map type without data loss

gptmacospartitioning

I have en external HD that I use with OS X. When I bought it, it was formatted for Windows (NTFS). I deleted the NTFS partition and created a new HFS+ partition. Then I copied 500 GB of material over.

Everything works fine when the external HD is directly connected to a Mac, but if I want to share the external HD by connecting it to a Time Capsule, I get an incorrect error message about the HD needing repair.

The problem is that the partition MAP is still in Windows format. For Time Capsule to recognize the disk, the partition map has to be in Guid Partition Table (GPT) format.

To do this, I have to delete the existing partition map. Will this also delete the underlying partition contents?

Best Answer

It's difficult to know for sure. Even if software promises not be destructive when changing the partition table, I still wouldn't take the chance without having a full backup of the drive first. Especially with a major operation such as change the partition table format, I'd worry that there's an even greater chance of data loss.

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I tried various other search terms and found that this question is actually a duplicate of this one: OS X 10.6 Snow Leopard no longer mounting an external USB drive.

Here's what I did to fix:

Download and install GPT fdisk

Run gdisk and select problem drive:

~% sudo gdisk
GPT fdisk (gdisk) version 0.8.1

Type device filename, or press <Enter> to exit: /dev/disk3
Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with protective MBR; using GPT.

Look at the partitions using the p command:

Command (? for help): p
Disk /dev/disk3: 1953525168 sectors, 931.5 GiB
Logical sector size: 512 bytes
Disk identifier (GUID): ABFA9105-73F4-4627-9890-9DECC55E86AC
Partition table holds up to 128 entries
First usable sector is 34, last usable sector is 1953525134
Partitions will be aligned on 8-sector boundaries
Total free space is 3389 sectors (1.7 MiB)

Number  Start (sector)    End (sector)  Size       Code  Name
   1              40          409639   200.0 MiB   EF00  EFI System Partition
   2          409640      1367597143   651.9 GiB   0700  Time Machine Backups
   3      1367599104      1953523711   279.4 GiB   0700  Basic data partition

In this case, it's partition 2 that's causing the problem. And the problem is that its type code is 0700, which means something other than HFS.

Use the t command to set the partition type to af00 - Apple HFS/HFS+.

Command (? for help): t
Partition number (1-3): 2
Current type is 'Microsoft basic data'
Hex code or GUID (L to show codes, Enter = af00): 
Changed type of partition to 'Apple HFS/HFS+'

Use the w command to write the stuff back to disk and quit.

Command (? for help): w

Final checks complete. About to write GPT data. THIS WILL OVERWRITE EXISTING
PARTITIONS!!

Do you want to proceed? (Y/N): y
OK; writing new GUID partition table (GPT).
Warning: The kernel may continue to use old or deleted partitions.
You should reboot or remove the drive.
The operation has completed successfully.

The HFS+ partition, and all of its contents, then became visible again to OS X.

Best Answer

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