Your Time Machine backup disk or partition should have no problem mounting on macOS Ventura or other versions. If it suddenly becomes grayed out in Disk Utility, you can try the following steps to fix it.
First, safely eject the external hard drive from your Mac and pull it out. Then reboot your Mac and reconnect the drive. Hopefully, the Time Machine backup partition is mounted now. If not, open Disk Utility, select the backup partition, and click the Mount button at the toolbar to manually mount it.
What else you can try is to force mount the partition in Terminal. Here's how to do it:
- Open Terminal from your Applications/Utilities folder.
- Enter the following command and press the Enter key.diskutil list
- Note down the disk identifier of the backup partition, such as disk2s2.
- Execute the following command to force mount the partition. (Replace disk_identiifer)diskutil mount /dev/disk_identifier
Suppose it still doesn't do the trick, then you can select the partition and click First Aid to check and repair disk errors.
If Disk Utility can't repair it, the last straw is to reformat the backup partition, which will erase all data on it. That being said, you need to recover the important files beforehand. To retrieve files from the unmountable backup drive on Mac, you'll need a professional data recovery tool like iBoysoft Data Recovery for Mac.
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External Hard Drive Not Mounting on Mac Computer, How to Fix?