Yesterday, I installed an external hard disk for my Time Machine — well, a 1.6Tb partition of a 2Tb hard disk. I am running a new iMac with 512Gb SSD. The internal drive is about half full.

After a day, I have about 5 backups, and I find that I only have 448Gb available.

I am still trying to get the other folder sizes (understandably, this is taking a long time), but the first backup folder is about 250Gb. At a rough estimate, this suggests that the subsequent backups are about the same size. At this rate, I will run out by tomorrow.

I understood that the Time Machine backups are sort of differential, and that subsequent backups are mostly links to the original backup files.

The question is: is the size, being mis-reported, or is that a correct estimate of the space used?


1 Answer 1


I would probably do two things:

  1. Let time machine run until it fills up the 1.6 TB and then see how long a windows you have. macOS will clean up the destination - purging old files as needed.
  2. Use tmutil compare or a tool like Backup Loupe to check what is taking the space. Other tools do a lousy job of showing you actual space usage in time machine.

  • Nice answer - I always think of Time Machine as simply "taking up all the space that's available" & make volume sizes appropriately.
    – Tetsujin
    Commented Feb 7, 2017 at 21:01
  • Thanks @bmike. I’ll try out your suggestions and get back as soon as I have more info.
    – Manngo
    Commented Feb 7, 2017 at 21:10
  • Sounds good @Manngo . A follow on question and link it here in a comment and I'll see it. Look for large database files - you'll want to exclude them and find another way to back them up or shard them (VMware and Parallels both have info on that for example)
    – bmike
    Commented Feb 7, 2017 at 22:10

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