MacOS – Fix a sometimes happening prohibitory sign ?

apfsbackupmacosmojavepartition

Well, I know there's an answer to a similar question here: Getting a ? 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?

Best Answer

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.

Related Solutions

Cannot repair disk0s2 with mavericks utility disk on a SSD

If you have booted from other media and tried Apple's tools then the only thing(s) left are 3rd party tools.

My go-to is Diskwarrior and while some drives with a lot of files will cause it to fail (so I have heard) I've, personally, never had an issue with it. It has always worked and almost always fixed the problem.

Failing that TechTool pro and Prosoft Engineering are a couple of other places to look.

MacOS – How to fix Mac hard drive partition showing as FDisk_partition_scheme

Please try the following:

Get the disk identifier of your external 3 TB drive
```
diskutil list
```
Below I assume the disk identifier is disk6
unmount the disk:
```
diskutil umountDisk disk6
```

Overwrite the first 40 blocks:

sudo dd if=/dev/zero of=/dev/disk6 bs=512 count=40

Create a new gpt:
```
sudo gpt create /dev/disk6
```

Check the disk info with:

diskutil info /dev/disk6

Reassure yourself that the device block size still is 512 Bytes

You may also use

sudo gpt -r show /dev/disk6

If the gpt shows:

   start        size  index  contents
       0           1         PMBR
       1           1         Pri GPT header
       2          32         Pri GPT table

you have a disk & disk controller which report a logical block size of 512 Bytes. Please continue with the next step.

If the gpt shows:

   start        size  index  contents
       0           1         PMBR
       1           1         Pri GPT header
       2           4         Pri GPT table

you have a disk & disk controller which report a logical block size of 4096 Bytes. Please stop here and add a comment.

First rebuild the EFI entry with:
```
sudo gpt add -b 40 -i 1 -s 614400 -t C12A7328-F81F-11D2-BA4B-00A0C93EC93B /dev/disk6
```
Depending on the size of the disk and the system version differently sized EFI volumes are built if partitioned with Disk Utility: either one with the size 200 MiB or one with 300 MiB. Here it's obvious that your disk contains a 300 MiB EFI and probably 4096 bytes of unallocated disk space: (314598400-1024)/512=614448 (=Start block main volume) 614448-40-8=614400 (=size of EFI)
Rebuild your main volume with:
```
sudo gpt add -b 614448 -i 2 -s SizeOfVolume1 -t 48465300-0000-11AA-AA11-00306543ECAC /dev/disk6
```
The size of the main volume can be determined by the first (corrupted & old) entry of the second GPT table: (3000592961536/512)=5860533128 is its block number. Then the size is calculated by 5860533128-614448=5859918680 blocks. Since 5859918680 is dividable by 8 (4096 physical block size/512 logical block size) this is a good guess for the volume size.

The best guess is finally:
```
sudo gpt add -b 614448 -i 2 -s 5859918680 -t 48465300-0000-11AA-AA11-00306543ECAC /dev/disk6
```
The second best guess is:
```
sudo gpt add -b 614448 -i 2 -s 5859918672 -t 48465300-0000-11AA-AA11-00306543ECAC /dev/disk6
```
Probably your lost volume gets mounted now. Verify the volume with:
```
diskutil verifyVolume disk6s2
```
If necessary try to repair the volume.
```
diskutil repairVolume disk6s2
```

Since you moved the "corrupted" disk to a different case and disk controller the logical block size was modified. The old partition map is probably based on a logical block size of 4096 Bytes.

To recover the partition map in the old (4096b) case you would had to enter the following to restore the GPT (based on David Anderson's answer):

Create a new gpt:
```
sudo gpt create /dev/disk6
```

First rebuild the EFI entry with:

sudo gpt add -b 6 -i 1 -s 76800 -t C12A7328-F81F-11D2-BA4B-00A0C93EC93B /dev/disk6

Rebuild your main volume with:

sudo gpt add -b 76806 -i 2 -s 732457067 -t 48465300-0000-11AA-AA11-00306543ECAC /dev/disk6

the final partition map looks like this:

 sudo gpt -r show disk1
       start        size  index  contents
           0           1         PMBR
           1           1         Pri GPT header
           2           4         Pri GPT table
           6       76800      1  GPT part - C12A7328-F81F-11D2-BA4B-00A0C93EC93B
       76806   732457067      2  GPT part - 48465300-0000-11AA-AA11-00306543ECAC
   732533873       32768         
   732566641           4         Sec GPT table
   732566645           1         Sec GPT header

Based on the 4096b part this "retranslates" after installing the disk in a 512b logical block size case to:

Create a new gpt:
```
sudo gpt create /dev/disk6
```