Today I needed to securely wipe an old laptop in preparation for computer recycling. The Mac Book Pro was running 10.6, so it had no built-in recovery partition. I did have another Mac running 10.11 and a spare SD Card laying around. So I decided to build a bootable recovery SD Card. It takes just a few minutes to do.
First, get a sense of what you’re working with by using diskutil list. Below, disk0 and disk1 are
the CoreStorage volumes that make up the logical volume disk2. Finally, disk3 is the target SD Card,
which has a FAT partition scheme, since it’s used in my camera. The things to note here are the
identifiers for “Recovery HD” and the target device.
% diskutil list
/dev/disk0 (internal, physical):
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *121.3 GB disk0
1: EFI EFI 209.7 MB disk0s1
2: Apple_CoreStorage Macintosh HD 121.0 GB disk0s2
3: Apple_Boot Boot OS X 134.2 MB disk0s3
/dev/disk1 (internal, physical):
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *1.0 TB disk1
1: EFI EFI 209.7 MB disk1s1
2: Apple_CoreStorage Macintosh HD 999.3 GB disk1s2
3: Apple_Boot Recovery HD 650.0 MB disk1s3
/dev/disk2 (internal, virtual):
#: TYPE NAME SIZE IDENTIFIER
0: Apple_HFS iMac +1.1 TB disk2
Logical Volume on disk0s2, disk1s2
XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX
Unlocked Encrypted Fusion Drive
/dev/disk3 (internal, physical):
#: TYPE NAME SIZE IDENTIFIER
0: FDisk_partition_scheme *32.0 GB disk3
1: Windows_FAT_32 32.0 GB disk3s1
The first real step is to unmount the target volume and format it as HFS+, so that it can be used as a Mac drive.
% diskutil unmount /dev/disk3
% diskutil erasedisk jhfs+ Make_Recovery disk3
Started erase on disk3
Unmounting disk
Creating the partition map
Waiting for the disks to reappear
Formatting disk3s2 as Mac OS Extended (Journaled) with name Make_Recovery
Initialized /dev/rdisk3s2 as a 30 GB case-insensitive HFS Plus volume with a 8192k journal
Mounting disk
Finished erase on disk3
The format operation mounted the newly minted disk, so unmount it.
% diskutil unmountdisk /dev/disk3
Now it’s time to locate the bootable recovery material that will be the source for the SD Card image. This is located on the hidden “Recovery HD” partition of your Mac. Mount it, using the identifier located above.
% diskutil mount /dev/disk1s3
Volume Recovery HD on /dev/disk1s3 mounted
% ls -l /Volumes/Recovery\ HD/com.apple.recovery.boot
total 991328
-rw-r--r--@ 1 root admin 1984 Sep 17 04:17 BaseSystem.chunklist
-rw-r--r--@ 1 root wheel 485094905 Sep 17 02:49 BaseSystem.dmg
-rw-r--r-- 1 root wheel 3213 Aug 25 19:07 PlatformSupport.plist
-r--r--r-- 1 root wheel 477 Sep 17 01:39 SystemVersion.plist
-rw-r--r-- 1 root wheel 604728 Oct 3 16:37 boot.efi
-rw-r--r-- 1 root wheel 365 Oct 3 16:37 com.apple.Boot.plist
-rw-r--r-- 1 root wheel 21837607 Sep 17 01:37 prelinkedkernel
The BaseSystem.dmg is the recovery image. Use the asr(8) command to copy it to the SD Card,
erasing its contents.
% sudo asr restore --source /Volumes/Recovery\ HD/com.apple.recovery.boot/BaseSystem.dmg --target /dev/disk3s2 --erase
Password:
Validating target...done
Validating source...done
Erase contents of /dev/disk3s2 ()? [ny]: y
Repartitioning target device...done
Retrieving scan information...done
Validating sizes...done
Restoring ....10....20....30....40....50....60....70....80....90....100
Verifying ....10....20....30....40....50....60....70....80....90....100
Next, make it bootable by using asr to change the volume type.
% sudo asr adjust --target /dev/disk3s2 --settype Apple_Boot
Fsck /dev/disk3s2 ....10....20....30....40....50....60....70....80....90....100
Adjust completed successfully
Finally, unount the Recovery HD.
% diskutil unmount /dev/disk1s3
Volume Recovery HD on disk1s3 unmounted
You now have a bootable recovery drive. You can hold down Option during a reboot to select it as the boot volume on a Mac.