Fix a sometimes happening prohibitory sign Well, I know there's an answer to a similar question here: Getting a &#128683; sometimes on boot, but this is slightly different.

My system is  running on external drive. Actually on an external drive partition. After I installed Waves plugins I encountered this error for the first time. So I booted in my main system on internal drive and used repair on that partition by sudo diskutil mountdisk force /dev/disk2 because partition didn't show up. That didn't help. So I used diskutil repairvolume /dev/disk2s4 and after repairing it helped and I was able to boot normally. It happened twice and each time at firsrt getiing  and after repairing a volume I was working fine (I'm running on this system now).

I tried reinstalling macOS – did nothing. I have an old backup that's lacking some files. So I guess, if I backup now it's going to backup the whole drive including system folders errors. Any ideas how to fix this without erasing/restoring onanism?
When you get a prohibitory sign when you attempt to boot, it means your Mac can't find any bootable media.

This is the EFI firmware searching for a device to boot from and not having any success.  It has nothing to do with the file system, so First Aid or even reinstalling will not fix the problem.

The most likely cause of this problem is a failing drive.  You have to get a new one.


  I have an old backup that's lacking some files. So I guess, if I
  backup now it's going to backup the whole drive including system
  folders errors.


That's no reason to not attempt a backup.  Your objective right now should be to backup your data and if it causes an error, so be it.  Losing a few files is far less damaging than losing all of your files.