Cloning an OSX disk using Linux, dd, and parted

Tonight I needed to upgrade a customer’s Macbook. She had a 160GB HDD, and wanted to upgrade to a 500GB HDD. No problem, right? Slap the old drive and the new one into an Ubuntu Linux workstation, dd if=/dev/sdc bs=4M conv=sync,noerror | pv -s 160G | dd of=/dev/sdd bs=4M and, 45 minutes or so later, we’re good to go.

Well, not quite. It turns out that, unlike Windows, Linux, or FreeBSD, OSX doesn’t really know how to deal with a partition TABLE that says it’s smaller than the physical disk is – so the cloned larger drive boots, and you can get into Disk Utility just fine, and it shows the drive as being 500GB… but when you try to expand the partition to use the new space, you get the error MediaKit reports partition (map) too small. Googling this error message leads you to some pretty alarming instructions for booting into an OSX install DVD, hitting the Terminal, and manually deleting and C-A-R-E-F-U-L-L-Y custom recreating the gpt partition table. Scary.

But no fear – you were using Linux in the first place, remember? Turns out, Linux’s parted command will automatically fix the gpt table. So after cloning the disk with dd, all you need to do is this:

root@box:/# parted /dev/sdd
GNU Parted 2.3
Using /dev/sdd

Welcome to GNU Parted! Type ‘help’ to view a list of commands.
(parted) print
Error: The backup GPT table is not at the end of the disk, as it should be. This might mean that another operating system believes the disk is smaller.
Fix, by moving the backup to the end (and removing the old backup)?
Fix/Ignore/Cancel? Fix
Warning: Not all of the space available to /dev/sdd appears to be used, you can fix the GPT to use all of the space (an extra 664191360 blocks) or
continue with the current setting?
Fix/Ignore? Fix
Model: ATA WDC WD5000BPKT-7 (scsi)
Disk /dev/sdd: 500GB
Sector size (logical/physical): 512B/4096B
Partition Table: gpt

Number Start End Size File system Name Flags
1 20.5kB 210MB 210MB fat32 EFI system partition boot
2 210MB 160GB 160GB hfs+ Customer

(parted) quit
root@box:/#

All fixed. NOW, when you boot that disk into OSX, go into Disk Utility and resize the partition, it will Just Work.

… unless, of course, you’ve got errors in the filesystem, which my customer did. To fix these, I needed to boot to single-user mode in the Mac by HOLDING DOWN command and S while the system booted (holding command down and pressing S rapidly won’t do the trick!) to get into single user mode, then I could fsck the drive with fsck -fy. After THAT, I could resize the partition normally from Disk Utility. :)

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Jim Salter

Mercenary sysadmin, open source advocate, and frotzer of the jim-jam.

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