For context, I have a 1TB internal SSD with 350GB used, and a 4TB external HDD with just over 3TB used. I've been backing up to an 8TB external HDD for about 3 years with no problems until now.

My understanding of Time Machine backups is that they are incremental and what gets backed up are the changes. I also thought that it would delete old backups once the disk filled up. But, lo and behold, I ran into an issue where it "doesn't have enough space to back up."

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So, 2 questions. Shouldn't Time Machine delete old backups to make room for this new one? And why is it suddenly requiring 3TB, if it's just storing the changes? The only thing I can think of would be if I moved a bunch of files between drives, but I didn't.

  • Seems to be getting more common - see my answer on apple.stackexchange.com/a/351143/85275 – Tetsujin Feb 13 '19 at 7:50
  • Ah, thanks, probably should have posted this on the Apple stackexchange. Will try this out when I get home. – davidmerrick Feb 13 '19 at 18:00
  • Wish you luck - let us know how you get on. Tag any reply with @myname so I'm notified. – Tetsujin Feb 13 '19 at 18:02
  • @Tetsujin will do! – davidmerrick Feb 13 '19 at 18:26

For many years James Pond was the Time Machine expert. He published a site called 'Pondini'. Whilst James is no longer with us his family has agreed to allow the information from his site to be used.

You may like to look at the first item here: http://www.baligu.com/pondini/TM/FAQ.html

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