Final answer:
Checking Time Machine preferences and manually deleting the interrupted backup are the initial troubleshooting steps to resolve issues with an unfinished Time Machine backup. Avoid reformatting the drive as it should only be a last resort.
Step-by-step explanation:
If a Time Machine backup was interrupted and did not finish, it is not necessary to take drastic measures such as reformatting the drive or reinstalling Time Machine immediately. The first step in resolving this issue is to check Time Machine preferences to verify the backup status and ensure that Time Machine is set up correctly. If the backup is indeed stuck or problematic, you can attempt to manually delete the interrupted backup from within Time Machine itself - this often involves entering Time Machine, locating the 'in progress' backup, and removing it. If you continue to experience problems after these steps, consider other troubleshooting methods such as ensuring the external drive is correctly connected, checking the drive's health, or looking into console logs for errors. In rare cases, formatting the drive might be necessary, but this should be the last resort as it will erase all data on the drive.