Target Disk Mode is a handy boot utility that enables you to boot from a separate drive and transfer data from one Mac to another Mac. This can be used if your Mac won’t boot and you need to get files off of it. You would simply use a FireWire or Thunderbolt cable so that the two Macs are connected, and one of them appears as an external hard disk on the other. However, there are times that you aren’t able to see the target computer on the host computer in FireWire mode. Don’t worry. This post will show you why Target Disk Mode is not working and how to fix this issue.
Case 1: Firmware Password is turned on
If the Mac has a firmware password set, it will prevent access to your Mac’s startup disk unless you know the password, and your Mac will not even boot into Target Disk Mode. You need to turn off the firmware password before booting the Mac in Target Disk Mode.
Step 1: Reboot your Mac and hold down Command + R button to enter into Recovery Mode.
Step 2: From the utility window, click Utilities in the menu bar and select Firmware Password Utility or Startup Security Utility.
Step 3: Click Turn off Firmware and enter your firmware password to disable it.
Step 4: Quit the utility and then choose Apple menu > Restart.
Case 2: FileVault is enabled
You are suggested to turn on FileVault encryption. However, this will prevent you from accessing FileVault encrypted home directories over Target Disk Mode. To enter into Target Disk Mode, you need to disable FileVault first.
Before getting into Target Disk Mode, you can boot the Mac normally. Choose Apple menu > System Preferences, then click Security & Privacy. Select FileVault and click on the lock icon to make changes. Then enter an administrator name and password. Click Turn Off FileVault.
Case 3: Your Mac is secured with a T2 chip
If your Mac computer integrates with a T2 Security chip, the built-in SSD data is encrypted in hardware with a unique random serial ID for encryption. In a word, the drive can no longer be separated from a T2 chip quipped Mac computer or attached to another Mac to access the data because the original T2 chip is the only processor to decrypt data. Therefore, Target Disk Mode won’t work anymore. Under this circumstance, you can still restore lost data from Time Machine Backups. However, what if you didn’t set up Time Machine? Fortunately, Mac data recovery software is still in the market to handle data loss from a T2-secured Mac. At present, iBoysoft Data Recovery for Mac is the only one, as far as we know, that can recover deleted or lost data from Macs released in 2018 and later. Here is a detailed introduction about the T2 security chip and its data recovery process: https://iboysoft.com/mac-data-recovery/apple-t2-chip-data-recovery.html
Case 4: The target disk is not mounted/corrupted/upgraded
If the Mac you’re trying to connect in the Target Disk Mode doesn’t appear on the host Mac, it might be the target disk is not mounted. Here are steps to mount the disk and get rid of this issue:
Step 1: Enter into Target Disk Mode by pressing down T at startup. Click Apple Menu > System Preferences.
Step 2: Click the Startup Disk. Then click on the Target Disk Mode button to restart your Mac.
Step 3: The target disk should appear as a volume in the Disk Utility sidebar. You need to select that disk and choose File > Mount from the menu bar.
Other reasons why the target disk is still not mounting or available to your Mac after those steps are the out-of-date OS or corrupted startup disk. If the target disk is formatted for APFS, the other Mac must be running High Sierra or later to be compatible with the host Mac. In this case, you need to upgrade the macOS first on the other Mac. The problem persists due to the damaged target disk, and the other Mac can’t mount the startup disk as an external disk. In this case, you should ask for help from a computer technician or professional data recovery software to restore important files then.
Target disk mode is a good option to migrate files from one Mac to another Mac, but it has some limitations when you use it. This method especially won’t work if your Mac has a T2 chip or the boot drive is corrupted. If not, try to decrypt the firmware and FileVault first to enable the target disk mode.