If you can't select your startup disk or start up from it
Some third-party external devices use Option ROM firmware. To enhance system security, Mac computers with up-to-date software don't automatically load Option ROM firmware. As a result, your Mac doesn't see devices that have Option ROM firmware until you load the firmware by pressing Option-Shift-Command-Period at the Startup Manager window. Do this each time you want to start up from the device or from a startup disk connected to it.

If you're using an Early 2015 (or older) Mac model, you can disable this security feature and allow your Mac to load Option ROM firmware automatically. This removes an important protection against potential unauthorized access to your Mac by other people with physical access to it.

1. Open the Terminal app, which is in the Utilities folder of your Applications folder.
2. Type sudo nvram enable-legacy-orom-behavior=1, then press Return.
   To undo this command, enter sudo nvram -d enable-legacy-orom-behavior.

(If you're using a firmware password, the ability to load Option ROM firmware is disabled as an additional security protection.)"

Of the 3 users that posted they were affected by this - 2 said the above tips worked, 1 said it didn't.
(And for those that use the terminal method, if you ever reset the NVRAM you'll clear that setting.)

I'm also curious if this affects function of some other external devices (docks, etc.) - not just drive enclosures. (i.e. What other external devices use Option ROM firmware.)

I don't know if the security updates for 10.11.6/10.10.5 also had this change, as it's not listed in the doc on security fixes but I've seen no posts about the problem from pre-10.12.2 users.

"About AirPort Base Station Firmware Update 7.7.8 Information
Firmware update 7.7.8 is recommended for all AirPort Extreme and AirPort Time Capsule base stations with 802.11ac. This update addresses an issue that may prevent AirPort base stations from appearing in AirPort Utility when Back to My Mac is enabled.

For detailed information about the security content of this update, please visit: https://support.apple.com/kb/HT201222
(nothing related there at post time)
For detailed information on AirPort base station firmware updates, including instructions how to install the update, please visit: this page."

"About AirPort Base Station Firmware Update 7.6.8
Firmware update 7.6.8 is recommended for all Apple 802.11n Wi-Fi base stations including AirPort Express, AirPort Extreme and AirPort Time Capsule. This update addresses an issue that may prevent users from accessing their AirPort base station using Back to My Mac.

For detailed information about the security content of this update, please visit: https://support.apple.com/kb/HT201222
(nothing related there at post time.)
For detailed information on AirPort base station firmware updates, including instructions how to install the update, please visit: this page.
For more information on using AirPort products, go to the Product Support Page at: https://www.apple.com/support/airport/"

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