OSX Rescue Files fom non-booting system

Description

Apple's highly integrated design may leave data resident on a fully working disk, which is very difficult to extract from the system.  Critical data may need to be retrieved or removed from the disk, prior to presenting for  warranty claim or third party repair.  Warranty is a particularly thorny issue as disassembly, of an iMac especially, is not straightforward, may introduce further issues and may void the warranty.

Workaround

In some situations where OSX fails to boot completely, it may be able to access BSD single user (verbose) mode,  similar to Linux Rescue mode.

Enter single user mode

1. Power off the Mac
2. Hold the command [Apple] + [s]
3. Power on the Mac, keeping the keys pressed until the screen turns black and single user mode is entered

Prepare Rescue Media

1. Prepare a hfs formatted usb hard disk, or similar external rescue device.
2. Optionally, create a file or folder with an easily recognised name on the rescue device, to verify identification when the device is mounted.
3. Boot the Mac into single user mode, with the rescue device attached
4. Single user mode is entered with the system partition mounted in read only mode
5. To enable read/write enter
   mount -uw /
6. Run a file system repair,  by entering
   fsck -fy
7. Create a mount point for the rescue media, e.g.
   mkdir /Volumes/backup

Mount Rescue Media

The internal hard disk device label will be disk0, with partitions assigned 's' numbers sequentially, such as disk0s0 etc.  in most instances, the rescue device will be the highest disk number.  Be aware that the hfs format may carry a small hidden partition at at s0.  If in doubt, double check by removing the rescue device and observing the change in /dev

1. Locate the rescue device label, entering
   ls /dev | grep disk
2. Mount the rescue device using the command, mount -t hfs <dev file> <mount point>
   E.g. mount -t hfs /dev/disk1s1 /Volumes/backup

The rescue device should now be mounted read write in /Volumes/backup

Use rsync or whatever to copy files from the failed system to the rescue device