I'd love for someone to do the empirical test to see whether booting from the end-of-the-chain external Sony drive works or not, but it may well not. (I honestly just don't know.) Perhaps it said so in boldface letters in the original manual, but the first attempt at Googling for a copy of the HD-20's user manual came up empty.

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I feel like I tried this once upon a time, but I do not recall the outcome.

The HD20 Manual is quite clear on this -- the HD20 Startup disk must be in the internal drive. It goes on to outline procedures for using an external drive without powering up the HD20 -- you must startup from the HD20 disk in the internal drive before an external drive connected to an unpowered HD20 will be recognized.