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External Disk with exFat or APFS Cannot Write on Mac after Updating

After updating macOS from 10.13.3 to 10.13.4 or even 10.14, my external disk with exFAT or APFS can't write though the access shows "Read & Write". But It can write on another Mac with macOS 10.13.3, any tip？

Best Answered by

iBoysoft author Sherry Song

Sherry Song

Answered on Tuesday, August 5, 2025

Glad to solve your problems.  Assuming the disks are correctly formatted, either exFAT or APFS, they should be readable writable in your macOS versions, so have a try:

  1. Downgrade your macOS to the original version 10.13.3.
  2. Reset Permissions: Open Termianl, type the command to reset permissions(sudo chmod -R 755 / Volumes /YourExternalDisk).
  3. Reset NVRAM/ PRAM: Shot down your Mac and press the power button then immediately hold down Command, Option, P, and R until you see the Apple logo or hear the starup sound.

If all these fixes can't do any help, backup all data in your friend's Mac and use it in a new external drive.

 

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