How to delete the partition table on a USB flash drive

partitionusb

I have a 32Gb USB flash drive, and I flashed it with a bootable image. So, that process modified the partition table, and the drive now reports a size of 1Gb. Unfortunately, erasing or reformatting the drive doesn't fix it. From what I've read, I need to completely nuke the partition table. Usually, this would be very easy to do on a Linux machine, but I must do it on a Mac right now. The Disk Utility app doesn't seem to have an UI for this. Is there another tool or set of commands I could use?

macOS Big Sur 11.3.1

Best Answer

Since some recent version of macOS (High Sierra? more precision needed here), Disk Utility stopped presenting by default physical devices. Thus users are protected against erasing their disks by error. I would say that MacOS is evolving to hide more and more risky functions to protect users (which is easier than to teach them the truth).

To recover the usual vision of devices and volumes, one has simply to open the left top View menu and select Show All Devices. In fact this Show All Devices will show all physical devices (disks, USB flash drives, SSD…) but not truly all volumes.

From there, you will see that actually your drive wasn't shrunk, but just the unique volume on it was defined to be 1 GB. By fully erasing your physical drive, you will destroy its actual partition and create a new volume of 32 GB.

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You can use Unetbootin to copy the Linux ISO to 1.2GB partition(disk4s3).

If I were you though I would first rewrite the partition type as GPT instead of APM by running:

diskutil partitionDisk disk4 GPT fat32 Linux 10% ExFat d2 10% ExFat d3 80%

Then select the Linux partition on disk4 in UNetbootin to be the destination for the ISO copy.

Partition type suddenly FFFFFFFF-FFFF-FFFF-FFFF-FFFFFFFFFFFF, drive unmountable

You have to edit the partition table and modify the type of disk1s2 (and correct the type of disk1s3) to get your main data partition back and make Boot OS X invisible:

Log-in as admin
Open Terminal and enter
```
sudo gpt -r show /dev/disk1
```
to get an overview
Unmount disk1:
```
diskutil umountDisk /dev/disk1
```

Remove partition 2 and 3:

sudo gpt remove -i 3 /dev/disk1
diskutil umountDisk /dev/disk1
sudo gpt remove -i 2 /dev/disk1

Re-add the partition 2 and 3

diskutil umountDisk /dev/disk1
sudo gpt add -b 928144008 -i 3 -s 262144 -t 426F6F74-0000-11AA-AA11-00306543ECAC /dev/disk1
diskutil umountDisk /dev/disk1
sudo gpt add -b 409640 -i 2 -s 927734368 -t 53746F72-6167-11AA-AA11-00306543ECAC /dev/disk1

The correct partition type of a Boot OS X partition is 426F6F74-0000-11AA-AA11-00306543ECAC (like a Recovery HD partition) and 53746F72-6167-11AA-AA11-00306543ECAC for a CoreStorage partition.

Update: gpt behaves strange because the partition table contains an MBR instead of an PMBR - the MBR has to be replaced by a PMBR:

Reboot your Mac
Open Terminal and enter
```
sudo gpt -r show /dev/disk1
```
to get an overview and all partition sizes/types

rebuilding the GUID partition table and destroying the bogus MBR

diskutil umountDisk /dev/disk1
sudo gpt destroy /dev/disk1
diskutil umountDisk /dev/disk1
sudo gpt create -f /dev/disk1
sudo gpt add -b 40 -i 1 -s 409600 -t C12A7328-F81F-11D2-BA4B-00A0C93EC93B /dev/disk1
sudo gpt add -b 409640 -i 2 -s 927734368 -t 53746F72-6167-11AA-AA11-00306543ECAC /dev/disk1       
diskutil umountDisk /dev/disk2 
diskutil umountDisk /dev/disk1
sudo gpt add -b 928144008 -i 3 -s 262144 -t 426F6F74-0000-11AA-AA11-00306543ECAC /dev/disk1
diskutil umountDisk /dev/disk2
diskutil umountDisk /dev/disk1
sudo gpt add -b 929021952 -i 4 -s 39294976 -t EBD0A0A2-B9E5-4433-87C0-68B6B72699C7 /dev/disk1
diskutil umountDisk /dev/disk2
diskutil umountDisk /dev/disk1
sudo gpt add -b 968318976 -i 5 -s 8388608 -t 0657FD6D-A4AB-43C4-84E5-0933C84B4F4F /dev/disk1

Best Answer

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Partition type suddenly FFFFFFFF-FFFF-FFFF-FFFF-FFFFFFFFFFFF, drive unmountable

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