I can't find/see a way to remove Brains?
So you mean the SD card flashing with stock firmware from Bitmain can't able to replace your current firmware? Or you might just did the wrong way?
If the SD card do not work then try the guide from Braiins documentation. Here check this link below
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FLASHING A FACTORY FIRMWAREMy thinking that the older FW i have (you label it stock -pre brains) is a .tar file and not an image file like Brains.
Brains therefore i can easily flash via SD card, the .tar older FW requires command line or some other process to flash (i'm talking pre BrainsOS 2017 FW etc, the old type Bitmain Fw)
I honestly don't understand how to do it via the 'commands', i presume i use command prompt and manually enter details as it describes, plus i somehow create an image of the .tar file to insert in this command line?
Brains FW already is an image file, so it's easily created to put on an SD card.