My MacBook Pro 2017 (running Big Sur) is not booting. Instead, it is showing me a flashing folder with a question mark.

When I boot from an external ssd, I can see my data from my internal ssd, but I can't delete something from it.

When I'm in recovery mode, I can't reinstall the os because I don't have enough space, and in disk utility, and when I'm trying to erase my disk, it says: "A writeable disk is required (-69772)".

What can I do to erase my disk and reinstall the os ?

  • Can you boot in Recovery Mode (cmd+r) then from the top menu you choise Utility/Terminal then open a new terminal (cmd+n) and in this new terminal you type the command '' /Applications/Safari.app/Contents/MacOS/Safari. This open a Safari windows whith you can connect here and we can exchange. In the first terminal you past the command diskutil list internal and you can add the result in your thirst message.
    – user415185
    May 7, 2021 at 13:46
  • @Jean_JD check imgur.com/C5yYVeT
    – Vasile
    May 7, 2021 at 13:57
  • You have boot from external disk? You want erase the disk? Your data are saved?
    – user415185
    May 7, 2021 at 14:26

1 Answer 1


If you don't want retrieve your personal datas (I suppose they are saved) you can erase the entire disk and after reinstall Mac Os X. I suppose you have an usb key or an apple installer for install your Mac os system.

I refer to this : https://imgur.com/C5yYVeT

The command in the terminal is :

diskutil erasedisk apfs "Alex HD" gpt disk0

This will reinitialise the 128 Go SSD with an APFS Volume named Alex HD

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