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How to Fix SanDisk Flash Drive Read-only on Mac and Error 69877?

Hello! I have this SanDisk Ultra USB 3.0 Flash Drive. Suddenly, it became "read-only" so I cannot delete any files whatsoever. I tried to erase it using Disk Utility and it always shows me the same error "couldn't open device" using both normal boot and safe boot. I have not tried erasing it in Recovery Mode or Internet Recovery but I will. I also tried Terminal to erase disk and then it says Error: -69877: Couldn't open device. Any tips?

Best Answered by

iBoysoft author Eudora Liu

Eudora Liu

Answered on Wednesday, November 26, 2025

To fix the external flash drive read-only on Mac, you need to check the file system of your external flash drive.

If it is NTFS, you can read only on Mac. And you can use iBoysoft NTFS for Mac to help you.

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If not, there are many ways that may cause your external flash drive read-only, such as a physical write protection tab, a corrupted file system, etc. And you can use the following ways to fix it.

  • Unlock the physical write protection tab.
  • Repair the file system of your external flash drive.
  • Remove read-only attributes using CMD.
  • Format your external flash drive.

The error 69877 may be caused by the failure to unmount the disk because it is currently used by other processes, like FileVault. And you can take the below ways to fix it.

  • Check your drive's permissions
  • Unmount the drive in Terminal
  • Erase the drive in macOS Recovery
  • Erase the drive in Safe Mode
  • Disable FileVault temporarily
  • Erase the drive on a Windows computer
  • Zero out the disk

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