yosemite upgrade error couldn't unmount disk
Late-model Macs that do not feature a Firewire port can still be placed into Target Mode via the Thunderbolt port. This will require the use of a Thunderbolt cable between the two Mac computers for the connection and is available from Apple.

Models that do not support booting to Target Mode are:


iMac (Tray-Loading)
Power Macintosh G3 (Blue &amp; White)
iBook ("Clamshell")
Power Macintosh G4 (PCI Graphics)
MacBook Air (2008-2009)
MacBook (Unibody)


It's possible to mix-and-match ports, e.g., Firewire-to-Thunderbolt, so it is not necessary to have the same port types on both computers. Putting your Target Mode-compatible Thunderbolt-equipped Mac into Target Mode is done the same way as with older Firewire systems: Turn off the Mac, connect the cables then turn on the power and depress the T key until you see the Thunderbolt icon displayed on the screen. The drive should mount automatically on the other Mac, which should then make it possible for you to access the drive.

Note that in Disk Utility, you'll want to select the Thunderbolt-/Firewire-mounted drive rather than its partition when you go to erase it. When you do erase the volume, be sure to give it the same name as currently used. This ensures that existing Time Machine backups can be analyzed and appended to with the new OS installation rather than discarded as belonging to a different system. Since Lion, OS X has been pretty good about being able to figure out when a Computer Name (set in Sharing) and volume Name (set in Disk Utility | Erase) are the same from installation to installation.

Best of luck with it!