Time Machine Backup Issue

Recently, I discovered that my Time Machine backup to an external hard drive was no longer working. I kept receiving failure messages, but they did not clearly explain the reason for the failure. Eventually, I realized that the available space on the drive was insufficient.

The quickest way to resolve this issue was to erase all backup data on the drive and start a fresh backup from scratch.

Erasing Time Machine Data

The best way to erase Time Machine data from the drive is by using the Disk Utility tool, which wipes out all data and reformats the drive as needed. However, when I attempted to erase the drive, I encountered another failure message. The system reported that the process "mds_store" was preventing the operation (holding the drive).

Following advice from a Google search, I tried terminating the process via Terminal. However, even after killing the process, I encountered the same failure message when attempting to erase the drive again.

Finally, I found a solution! I restarted my Mac in Safe Mode by holding the Shift key during startup. Since Safe Mode runs with minimal processes, I was able to successfully erase the data on the drive by using Disk Utility.

Adding the Drive Back to Time Machine

Once I had the drive ready, another issue appeared—the drive did not show up in the list of available backup drives, preventing me from adding it back to Time Machine.

I recalled receiving a warning message about "Forgetting this drive as a Time Machine backup" when I initially removed the external drive from the Time Machine settings. It seemed that macOS had stored a hidden setting to prevent this drive from being used as a Time Machine backup again. Unfortunately, I wasn’t sure where to change this setting.

Fortunately, I noticed that another partitioned drive appeared as an available Time Machine option. This drive contained important data, such as my picture libraries and historical records. When I selected this drive as a Time Machine backup, the backup process started immediately. However, I did not want to use this drive for backups, so I stopped the process.

One thing I observed was that a Backups.backupdb directory was created on this data drive. I decided to copy this directory to my intended backup drive. To my surprise, after doing this, the drive finally appeared in the Time Machine settings as an available backup option!

Time Machine Permissions

Another interesting discovery was related to permissions on my Time Machine drive.

When I erased and reformatted the drive, the permission settings were as shown in the following picture:

These permissions were the same as those of my data drive. However, when the drive was set up as a Time Machine backup, the permissions changed to the following:

I noticed that I could no longer modify these permissions or add an admin user. If I wanted to copy or erase any data on the drive, I had to enter my admin passwordeach time.

Conclusion

After a long struggle, I finally managed to get my Time Machine backup working again!