Time Machine does partial back up

[rainbowcat]

Using a Seagate external 500GB hard drive, running latest macOS on MBA. When I plugged in the external hard drive it took quite a while to show up on the desktop, and Time Machine only backed up 1GB. Ran it again, and it backed up 38MB. Never had this problem before - Time Machine back ups took about 15 minutes and all was well. I thought when the hard drive was full the oldest backups were automatically deleted; do I need to do this manually? Or is something else going on here? Hard drive shows 7GB of free space.

 

[Slydude]

I assume from your post that the Time Machine drive is not always connected to your Mac correct? If so, read on.

The first time you connected the drive the backup was pretty large (1GB) because it had been some time since the last backup and Time Machine has to copy every file that has changed. If you ran it again shortly thereafter there would be a smaller amount needed to back up. Time Machine does not rewrite all of the files when it creates a backup. Here's how the process works.

1. On first run Time Machine backs up everything except files you told it to exclude.
2. On the next run time Machine scans looking for files that have changed If a file has changed a backup is made of that file. If a file is unchanged a link (pointer) is made to the original file. The file is not copied again. This results in updates being faster and taking up less space than the original backup.
3. When it gets short of Space time Machine will delete the oldest backup. Given the amount of data being copied you seem to have enough space. Some users seem to have ruined their bckups bu trying to manage this themselves.

Did the short backup (a few MB) take 15 minutes? If so, we may need to let Disk Utility check the drive for problems.

 

[rainbowcat]

Thanks Slydude! I thought Time Machine backs up the whole deal every time, not just the changes. I have it set to tell me when it deletes the oldest backup. The small backup took about 3 minutes, so I guess everything is copacetic. Tried to add to your rep, but you've helped me so many times that I have to "spread the love around"!

 

[Slydude]

Tried to add to your rep, but you've helped me so many times that I have to "spread the love around"!

No problem. It's the thought that counts. I have had that problem trying to give rep to a few members of the brain trust around here.

Glad that seems to have sorted things out for you. BTW don't feel to bad about misunderstanding how Time machine works. We seem to get quite a few posts from folks with the same misunderstanding.