I can hold down alt at boot to get the OFP login window, but once that is entered, the only option is to select the boot disk. There does not appear to be a way to tell it to enter target disk mode.
Open the Startup Disk pane of System Preferences and press the button for target disk mode.
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I also found that it is possible to get to target disk mode after entering the password and being presented with the boot disk choice(s). Simply pressing T will immediately enter Target Disk mode. I have not found that documented elsewhere.
How do I use a computer temporarily in Target Disk mode when OFP protection is enabled?
