Why storage size difference in MBP and backup drive?

[celliott]

So I have a 2015 13" MacBook Pro with a 500GB SSD running OS 10.14.6 Mojave. I hadn't done a backup for awhile so thought I'd take care of that. Ran CCC onto an external spinning HD formatted APFS just like the SSD. After the backup completed I noticed a big difference in the size reported on what was stored.

The backup only took up 174GB on the spinning disk while the SSD reported to be 267GB. What? I checked individual folder sizes between the disk and the SSD and they all proved to be almost identical in size. So, I reformatted the SSD and restored from the hard drive. The SSD now reports to hold 175GB. I'm running it right now and nothing is missing and everything is fully functional as it was before, only now the SSD is 100+Gb slimmer.

Anyone give me a clue or insights on what is going on? The trash was empty on each check, so it wasn't trashed files. I can understand some differences in size, but this is nuts.

[JoeH]

There are system area files such as the swap file that are not copied by CCC, at least at the default settings. They are "temporary" in the sense that they will be recreated on the fly if needed, and do not contain any data needed to operate your system after a restore. Over time they will take up space again.

[celliott]

Anyway to eliminate them during normal use? I mean, it would be nice to know what kind of storage space I have available. I don't remember something happening like that before to that extreme. Thanks for the insight.

[RAMd®d]

A restart will eliminate swap files.

There were/are apps that do that without restarting, but they’re not really necessary.

[btfc]

A Safe Boot will usually free up some room.

[Sarcany]

If you're making Time Machine backups then you also had a few GB of snapshots on your boot drive. I've seen the snapshots grow as large as 64GB on a Mac that hadn't been rebooted for awhile.