I've got an RB750Gr3 that had protected-routerboot enabled. I'm moderately certain it was running 6.39.2 with the latest Routerboot.

I hit the pinhole, and even though I gave it lots of time after this thing now appears to be a brick. It lights up but the beeper doesnt even sound anymore on power-on. I've confirmed my Netinstall setup is good by netinstalling several other routers with it, but this hEX does not show up. The USR light is completely dark, no amount of fiddling with the reset button gets it to do anything....

Anyone else seen anything like this? I do have another hEX I could try to reproduce this on, but I've already got one brick, I don't really want two...

My most recent config export off that box had the following settings:
/system routerboard settings
# Warning: memory not running at default frequency
set memory-frequency=1200DDR protected-routerboot=enabled \
reformat-hold-button=1m

This is what protected routerboot is supposed to do, isn't it?
Why did you enable it?

Protected Routerboot is supposed to zap the flash then boot Netinstall - it's not supposed to turn the router into an unrecoverable brick.

use the failsafe.

Remove the power.
Press and hold the reset.
Apply power.
Hold the reset for 300 seconds.
Once the router is reformatted, you will need to NetInstall the current routerOS to boot again.

use the failsafe.

Remove the power.
Press and hold the reset.
Apply power.
Hold the reset for 300 seconds.
Once the router is reformatted, you will need to NetInstall the current routerOS to boot again.

No go. I held the reset button, powered the router, held reset for over 5 minutes, released. 20 Minutes has now elapsed since then and the router has not appeared in netinstall. Neither the USR light nor the beeper has done anything at all.

RouterBOOT on this thing appears to be bricked.

But you have locked it down using the reformat-hold-button time!
That means you need to press it for THAT amount of time.
When you don't remember the time or it has some way not worked, you have essentially bricked your device.

But as far as I understand, that is the purpose of protected routerboot.
It is not something you should turn on because you think "protected is always good". No, it is a feature to prevent
theft and re-purposing of devices, e.g. "free loan router with your internet connection" devices being re-sold and
used for different purposes, or similar.
The intended users of this feature probably prefer bricked devices over devices they bought being re-sold or
taken over by competitors.

But you have locked it down using the reformat-hold-button time!
That means you need to press it for THAT amount of time.
When you don't remember the time or it has some way not worked, you have essentially bricked your device.

This is incorrect. A quote from the wiki (see https://wiki.mikrotik.com/wiki/Manual:R ... bootloader):

As an emergency recovery option, it is possible to reset everything by pressing the button at power-on for longer than reformat-hold-button time.

Even if reformat-hold-button time is forgotten, holding the reset button for more than 300s will allow you to perform reformat.

it is a feature to prevent theft and re-purposing of devices, e.g. "free loan router with your internet connection" devices being re-sold and
used for different purposes, or similar.

This is not correct either. The purpose of this feature is to protect the configuration of your device (including sensitive information like shared keys and passwords) from someone who may get physical access to your device. This has been discussed and confirmed by Mikrotik staff multiple times here on the forum.

Thank you very much for reporting, we found that there might be some "protected routerboot" issues on RB750Gr3 (hEX r3), and only on some specific backup booter versions. If you have any related problems, please write to support@mikrotik.com

Updated bakup BIOS for RB750Gr3 (hEX r3) can be found here:
https://www.mikrotik.com/download/share ... _mmips.dpk

Thank you very much for reporting, we found that there might be some "protected routerboot" issues on RB750Gr3 (hEX r3), and only on some specific backup booter versions. If you have any related problems, please write to support@mikrotik.com

Updated bakup BIOS for RB750Gr3 (hEX r3) can be found here:
https://www.mikrotik.com/download/share ... _mmips.dpk

ON hEX RB750Gr3

Thank you very much for reporting, we found that there might be some "protected routerboot" issues on RB750Gr3 (hEX r3), and only on some specific backup booter versions. If you have any related problems, please write to support@mikrotik.com

Updated bakup BIOS for RB750Gr3 (hEX r3) can be found here:
https://www.mikrotik.com/download/share ... _mmips.dpk

ON hEX RB750Gr3

New main BIOS firmware will be added in the next RouterOS releases.

I have update the instructions on old post:
viewtopic.php?f=2&t=94303&p=580430#p580430