"Time Machine backs up only the files that changed since the previous backup.", but what exactly is meant...

It’s called ‘Time Machine’ because nothing is deleted from the backup, at least until the backup disk you’re using is full. At that point, the ‘oldest’ files (those deleted or modified versions) are successively deleted.

In your example, A-E are all there in the backup. Plus, if you modified B and D you can go back to the earlier versions of those files if you need to.

As I understand your questions, at least some of them appear to be predicated on a misconception: We do not delete TM backups.

As for what items TM backs up, it backs up and retains as many versions of them as the capacity of the backup drive will accommodate. It periodically "thins" what it retains on a daily, weekly, and monthly basis, the intent being that the older an item becomes, the less likely the need to restore it.

In your example, files A, B, C, D and E and as many previously backed up versions of them are all retained, unless of course you delete them from the source volume, after which they become candidates for deletion.

You do not micro-manage the complex TM data base...

The main advantage of TM is that it creates recursive backups—enabling you to restore a specific file or the entire drive— from a certain point in time; this through the TM interface

it is not considered an archival backup for this reason.

If you value your user data

3-2-1 Backup Strategy: three copies of your data, two different methods, and one offsite.

More than one device, more than one backup methodology.

Time Machine is not an archive. It is an extremely capable Backup solution.

You do not, and in most cases, CAN NOT delete anything from a Time Machine backup drive.

because Time Machine includes complex database capabilities, Time Machine saves, and can re-create, ANY Moment in time for which it still holds backup files.

Example: a user posted that they had installed software that messed up their files. it was installed mid last week. They had done an ordinary restore to recover, but that restored to yesterday, and the problem was still there.

Readers were able to guide them to Time Machine.APP (not to be confused with Time machine preferences). This allowed them to look at the state of the major folder involved, back in time through each previous backup, until right BEFORE the bad software was added -- mid last week-- and restore as of that moment. They were thrilled.

coatli wrote:

hi mac experts!

im trying to understand the specifics of time mcahine.

Apple help says:

"Time Machine backs up only the files that changed since the previous backup.",

but does this mean that if i were to delete all backups prior to the most recent backup, all my files would be deleted except for the recently-changed one?

You can’t delete backups

For example say i have files A, B, C, D and E on my mac. So my first backup has A, B, C, D and E. But then i make some changes in B and in D. So the second backup only backs up B and D. But does that mean the second backup only CONTAINS B and D?

Sort of. They will be the only new files in that backup. The backup also has references to all the other files that didn’t change.

Or does it still CONTAIN A, B, C, D and E? And therefore if i delete the first backup will files A, B, C, D and E still remain in the second backup?

Again, you cannot delete backups.

The original backup will have all the files. Subsequent backups will reference the storage of the unchanged files. If it were actually possible to delete backups, then you would have deleted the files, but it also doesn’t work like that.

And if i choose to exclude some files from my future backups, say i exclude A and E, will the future backups still CONTAIN A, B, C, D and E, but just without the A and E being updated?

A and E will still be stored.

For a clear explanation of EXACTLY how Time Machine works its magic, nothing beats the late James Pond's illustrated guide, still completely applicable and still available from Mirror sites many years after his death.

This section of the guide is six pages, linked with the "Next" button at the bottom of each page.

How Time Machine Works its Magic

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