Disk Utility cannot erase USB Drive

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Level 2

304 points

Disk Utility cannot erase USB Drive

I'm trying to reformat a USB thumbdrive that I'd previously been using as a Linux boot device.

Unfortunately Disk Utility cannot erase the device.

When I insert the drive, I get a warning that it's not readable:

So I click "Initialize..." which launches Disk Utility. In here, it shows up as "disk4s2" (the disk number can vary if I try a different MacOs device), which doesn't look right to me.

When I try to erase the drive, I get the error "Couldn't modify partition map."

There's no write-protect switch on the thumbdrive. How can I fix this?

iMac with Retina 5K display, macOS High Sierra (10.13), 4GHz Intel i7, 32GB RAM, 3TB Fusion

Posted on Oct 25, 2017 10:17 AM

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Answer 1

Top-ranking reply

CyberGene

Level 1

26 points

Nov 30, 2017 3:26 PM in response to m0thr4

There's a much simpler solution. By default Disk Utility shows only volumes. Apparently this is a volume and can't be erased due to unsupported partitioning scheme. So, from the View menu choose "Show All Devices" and then you will be able to see the actual USB device that holds this volume and erase it.

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Answer 2

m0thr4 Author

Level 2

304 points

Oct 25, 2017 1:24 PM in response to m0thr4

Found the answer - diskutil at the command line.

My USB drive shows up in "diskutil list" as:

/dev/disk4 (external, physical):

#: TYPE NAME SIZE IDENTIFIER

0: FDisk_partition_scheme *62.7 GB disk4

1: 0xEF 9.2 MB disk4s2

I think the problem is the partitioning scheme. To erase the device completely, I just ran:

diskutil eraseDisk free EMPTY /dev/disk4

diskutil list

/dev/disk4 (external, physical):

#: TYPE NAME SIZE IDENTIFIER

0: GUID_partition_scheme *62.7 GB disk4

1: EFI EFI 209.7 MB disk4s1

Much better... it now has a partitioning scheme MacOs can work with. At this stage, you won't see it showing in the Disk Utility program, but fortunately formatting it is trivial. I decided to use ExFAT, and name the drive USB64:

diskutil eraseDisk ExFAT USB64 /dev/disk4

/dev/disk4 (external, physical):

#: TYPE NAME SIZE IDENTIFIER

0: GUID_partition_scheme *62.7 GB disk4

1: EFI EFI 209.7 MB disk4s1

2: Microsoft Basic Data USB64 62.5 GB disk4s2

And now I have a fully working USB drive again!

TIP: if ExFAT isn't your thing... there are of course other filesystems available. To see them, run:

diskutil listFilesystems

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Answer 3

dialabrain

Level 10

126,437 points

Oct 25, 2017 10:32 AM in response to m0thr4

Try erasing it again. For some reason some flash drives fail the first time.

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Answer 4

m0thr4 Author

Level 2

304 points

Oct 25, 2017 12:44 PM in response to dialabrain

dialabrain wrote:

Try erasing it again. For some reason some flash drives fail the first time.

I've tried multiple times both on my iMac and Macbook... same result each time.

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