How exactly do Time Machine back ups work?

[dellavoce]

So, I just set up a new hard drive to do Time Machine backups of my laptop. It has been running for a very long time, but the back up is almost 600 GB, so I guess that's not unreasonable. However, Time Machine is set to back up every hour! My question is, is each Time Machine back up going to be nearly 600GB?! After 5 hours, my 3 TB hard drive will be full. And it can't possibly back up each hour because this back up is taking hours.

I'm hoping that the reason it's taking so long is that it is a very large back up, but that each subsequent back up will be relatively fast since it will only note the differences between back ups?

 

[nightlong]

The first time it is backing up everything, after that it is backing up only changes. But, if your hard disk is full after the first backup to Time Machine, that is a problem. You'll either need to get a bigger external drive or split your storage. Time Machine needs space for the subsequent backups, and it will begin to delete from the back when the disk is full.

Edit: I meant to add ... the way I back up is over several disks. I prefer this as it reduces the risk of losing my data.

I keep all media (movies, photos, music, plus archive files) on one large drive, and my favourites of that also on a smaller portable drive. On another small portable drive a Time Machine backup of just what is on my laptop, on a larger drive at home that is partitioned there is another TM backup and also a Carbon Copy Cloner (bootable) backup. Then another TM backup on another drive for my iMac.

 

[NativeTxn]

The first back up takes a while depending on the size it is having to copy. After that, the subsequent backups really only need to copy new files, apps, etc. that have been changed since the last update.

Therefore, if you have not done much since the first (most recent) update, then the subsequent updates will likely only take a couple of minutes.

If you later moved 40 GB of photos or videos on, then that subsequent backup could take a while, but generally, after the first backup, the following ones will be rather short.

 

[glenthompson]

One thing that can cause problems with time machine backups is Parallels virtual machines. Time machine will always backup the entire vm which uses disk space quickly. Best to exclude them from the backup.