Final answer:
First, check Time Machine preferences to ensure the external drive is selected for backups. If issues persist and no errors are found, you might need to reset Time Machine or reinstall its software. Reformatting the drive or buying a new one are measures to take only when other troubleshooting steps have failed.
Step-by-step explanation:
If your Time Machine is no longer backing up to your external drive, even though the drive mounts correctly on other computers, there are some steps you can take to troubleshoot this issue before considering more drastic measures, like reformatting the drive or buying a new one.
Step-by-Step Explanation
Check Time Machine preferences: The first step is to ensure that Time Machine is set up correctly. Go to System Preferences, click on Time Machine, and make sure that the external drive is selected as the backup disk.
If the drive is selected but backups are not occurring, check to see if there are any error messages or if the drive is full. Time Machine requires free space to create new backups.
If the drive has plenty of space and no errors are displayed, try turning Time Machine off and on again within the preferences pane to reset the backup process.
If none of these steps resolve the issue, you may consider whether the Time Machine software itself might be the problem. Reinstalling Time Machine or updating macOS to the latest version might help.
If after these attempts your Time Machine is still not functioning, it might be necessary to reformat the external drive, but this action will erase any existing data on the drive, so it should be done as a last resort.
Lastly, purchasing a new external drive should only be considered if you've confirmed that your current drive is indeed failing and it's not just an issue with settings or software.