Booting from another mac's hard drive

Solution 1:

An installed OS X can be specific to older models. People that administer Macs professionally call this situation "forked" and no one likes to be "forked" by surprise.

Boot the new Mac to the setup assistant (or if you have made an account there already consider making a new test user account and backing up everything on the new mac that you need to save). Then you can run the Migration Assistant and put the old Mac in Target Disk mode (TDM) instead of booting the old OS from the new Mac.

TDM works over USB-C, FireWire or Thunderbolt ports, you can connect them so that one of them appears as an external hard disk on the other. This is called “target disk mode.”

1. Connect the two computers with one of the above cable types.

2. Start up the computer to be used as a disk in target disk mode:

   • If the computer is off, start it up while holding down the T key.

   • If the computer is on, choose Apple menu > System Preferences, click Startup Disk, and then click Target Disk Mode.

     When the computer has started up, a disk icon appears on the desktop of the other computer.

3. Transfer files by dragging them to and from the disk.

4. Eject the disk by dragging its icon to the Trash.
   While you drag, the Trash icon changes to an Eject icon.

5. On the computer you used as a disk, push the power button to shut it down, and then disconnect the cable.

Source: Apple KB - https://support.apple.com/kb/PH10725