“Couldn’t Modify Partition Map” Error and How to Fix It

couldn't modify partition map

If you’re working with a drive and you keep getting the “Couldn’t Modify Partition Map” error, that typically means the storage device got corrupted or something went wrong with the file system. This makes the entire device unreadable to all but very specific software.

In this article, we explore three ways to deal with this error. We included step-by-step instructions as well as accompanying screenshots so you can safely work on your drive without worrying about your files (more on this later). Read on!

What is a Partition Map?

A “partition” is a segment (called a “region”) in a disk that is treated as an individual storage unit. A disk can have multiple partitions that show up as individual storage units when connected to a computer – even if, physically, it’s just a single disk.

A “partition map” defines how partitions are organized on a disk. It contains crucial information about the volume. So when it gets corrupted by a virus attack or a power surge, for example, it becomes unreadable.

How to Recover Deleted Data from a Problematic Device

Because of the nature of volume and partition corruption, the solution to this problem usually includes erasing data. There’s just no way around it. If you have important files on your drive or partition, recover your data first. A super simple way to do this is by using a data recovery tool like Disk Drill. Even if your operating system can’t read your volume – Disk Drill can.

Disk Drill is a paid tool – however, you can preview unlimited files for free (even videos). This is important because if you find that even recovery software can’t save your data, your drive might be physically damaged. Any further recovery attempt can worsen the problem and delete your data forever. In this case, you’ll have to send your drive to a professional data recovery center.
  1. Download the free Mac version of Disk Drill from the Cleverfiles website and install it.
  2. Connect your device to your Mac.
  3. Open Disk Drill and select the problematic USB, SD card, or drive.
    disk drill drive selection menu
  4. Click the “Search for lost data” button.
    search for lost data
  5. Let Disk Drill complete its scanning process. You can also use this menu to start going through recovered files by type, and you can even review a lost partition specifically. For this article, we’ll click the “Review found items” button at the top right corner of the window so we can look through all recovered files.
    disk drill file type menu
  6. You can preview as many files as you want. Just click on the file for its preview to appear in the separate window. Once you’ve selected the files you want to recover, click the “Recover” button.
    disk drill recoverable files menu with a sample image preview
  7. You’ll be greeted by a pop-up. You can choose a safe drive or choose specific folder using the appropriate button. Make sure you choose a place other than the affected drive. Click the “Next” button. Congratulations, you have successfully recovered your data from your SD card, USB, or hard drive!
    choose the destination folder

How to Fix “Couldn’t modify partition map” Error

Method 1: Use Disk Utility’s First Aid Feature

If you encounter the error “Couldn’t modify partition map” on your SD card or flash drive, the first thing you can try is Mac’s built-in First Aid feature within the Disk Utility app. First Aid can check and repair file system errors, which can solve minor issues before they become bigger problems.

  1. Open Disk Utility by using Spotlight Search (CMD + Space) or by opening Finder > Applications > Utilities > Disk Utility app.
    utilities folder within applications folder
  2. Select the problematic drive.
    disk utility window
  3. On the top right corner of the Disk Utility window, click the “First Aid” button.
    first aid button

Method 2: Use Disk Utility to Format the Drive and Create a New Partition Map

For this method, we’ll again be using Disk Utility. If First Aid didn’t work, we can use the app to reformat the device – this will inevitably delete your data. Make sure to conduct a recovery beforehand!  Formatting a device will allow you to create a new partition map for a completely new partition.

  1. Open Disk Utility by using Spotlight Search (CMD + Space) or by opening Finder > Applications > Utilities > Disk Utility app.
    spotlight search popup
  2. Click the “View” button on the top left corner of the Disk Utility window and click “Show All Devices.”
    show all devices option
  3. Select the storage device (not the partition) and click the “Erase” button at the top of the window.
    disk utility erase button
  4. Change the name of the device to whatever you want, and choose MS-DOS (FAT) for the format, and “Master Boot Record” for the scheme. Click the “Erase” button. This overwrites the old partitions and creates a new one, along with a new partition map.
    partition properties and the erase button
  5. Wait for the erase process to complete, and click “Done.” You now should be able to use your device.

Method 3: Use the Terminal App to Format the Drive Create a New Partition Map

If the Disk Utility app didn’t work, you can try erasing the device and creating a new partition map using the Terminal app.

  1. Connect your USB flash drive, SD card, or external hard drive to your Mac.
  2. Open the Terminal app by using using Spotlight Search (CMD + Space) or by opening Finder > Applications > Utilities > Terminal app
    terminal app
  3. To find your Disk’s identifier, type the following command and press return:
    diskutil list

    terminal disk identifier

  4. Once you’ve identified your disk, type the following command and press return:
    diskutil eraseDisk MS-DOS “disk name” /dev/”disk identifier”
    Change “drive name” to whatever you named your storage device, and “disk identifier” according to the identifier you found in Step #3. For example, here’s what I would type on the Terminal app:

    diskutil eraseDisk MS-DOS SAMSUNG BAR /dev/disk4s1

If successful, you’ll lose all your data but you will be able to use your device normally again.


With the error “Couldn’t Modify Partition Map”, the drive is usually fixable. Unfortunately, that often means erasing the data stored in it. Make sure you use data recovery software like Disk Drill beforehand if you have important files you want to salvage. Even better, use Disk Drill or even the free Time Machine to routinely back up your files.


If this partition cannot be modified, it means that the partition map (or the entire partition or even the entire disk) was corrupted, or certain information on the partition is missing. The partition then becomes unreadable, and your OS can’t interact with it as usual.
If you want to change the partition map, you’ll have to reformat the entire disk. There’s no way to isolate this modification for a single partition. Before reformatting, make sure to recover and back up your important files.
    1. Recover and back up your data with data recovery software like Disk Drill.
    2. Try Disk Utility’s First Aid feature to repair the partition map on your Mac.
    3. Reformat the disk. Again, recover and back up your data first.
Alejandro Santos
Chief Writer
Alejandro is Macgasm’s Chief Writer and Apple ecosystem enthusiast. He pens the majority of troubleshooting guides and software reviews for this website, tapping into his love for technology and extensive background in technical writing. He started his career by helping… Full Bio