Target Disk Mode
Target Disk Mode is a feature of Mac laptops that makes Mac hardware issues much easier to troubleshoot. Basically, a laptop in TDM acts like an external hard drive, which you connect to any other computer using a Firewire cable.
- Power up (or shutdown and restart) the Mac.
- Immediately after powering on, hold down the t key until the Firewire icon appears on the screen (looks like a windmill).
- Now connect the Mac to another Mac using a Firewire cable. The next section explains what you can do with your Mac in TDM.
Uses for TDM
As an external hard drive
When you plug your TDM-Mac into another Mac that is already booted up and running, the TDM-Mac's hard drive will appear in the Finder as an external drive. You can browse it, and manage, copy, and move files as you could with any external drive. This option can also be used for data recovery.
As a bootable device
This is a bit more confusing: you use the computer in TDM as a boot device to start up a Mac system using the TDM hard drive but a different laptop's hardware.
To boot Mac A from Mac B's hard drive using TDM:
- Shut down Mac A.
- Boot Mac B into TDM and connect the two computers with a Firewire cable.
- Power up Mac A and immediately hold down the Option key until a window appears showing boot options.
- Select Mac B's hard drive (may be called "External hard drive" or something like that).
- Mac A will boot using Mac A's hardware but Mac B's hard disk and software.
This is really useful if you have a problem and you don't know if it's a hardware issue or a software issue. There's also some strange feature of Mac where Mac A's internal hard drive will spin more slowly - and hence, a damaged hard drive won't wear itself out as quickly - when Mac A is booting to Mac B's hard drive using TDM.
=> What are the relative merits of using this option vs. booting from a TDM drive?