When your backup external drive is full using Time Machine, is it possible to lose important old files?

So, if I understand you right, your most recent backup on Time Machine is a complete backup of your iMac's present contents?

Yes, at the time that backup was performed, excluding any items you may have chosen to exclude.

Hey, JazzmanJohn,

I understand that you had some questions about how Time Machine works, and I'm happy to clarify as much as I can.

You're correct that Time Machine will automatically delete old backups as your disk gets full. The system is designed to automatically delete the oldest backups first once that disk is full. The best way to prevent that would be to simply use a larger drive with Time Machine so that you can preserve your history for as long as possible.

This page here has additional details on that: If the Time Machine backup disk for your Mac is full.

If you do opt to purchase a new, larger drive, you can always bring over your backup history as is discussed in the article linked above. Another good article to bookmark, with additional info, can be found here: Back up your Mac with Time Machine.

Cheers.

Time Machine is NOT an archival backup system. It is essentially a "clone" of your current hard drives.

If you delete a file from your hard drive, it will eventually be deleted from the Time Machine backup. That file becomes part of the "old backups" that might be deleted when space is needed.

It seems to me you have conflicting goals. You originally expressed concern that older backups will be erased when Time Machine needs to reallocate the space they occupy for newer backups. That question has been answered. Now you are asking if it's really necessary to preserve your existing backup history, and only you can answer that question.

Bear in mind Time Machine will always retain, at an absolute minimum, one and only one complete backup of its source volume's present contents. It will also retain backups predating that one within that backup device's capacity. There is no practicable way to predict exactly when Time Machine will require that additional space, and it won't inform you until after those older backups are deleted. When it does that (assuming you selected the option to inform you) TM will inform you when its backup date range has been shortened.

If I do get a new, larger external drive, is it really necessary to preserve my existing backup history? If you just back up your entire iMac and go forward from there, isn't that enough?

It is for my particular requirements. Anyone else's requirements, including yours, may be different.