Announcement: Upgrade to macOS Mojave
This seems strange. I'm trying to back up my computer which has 120 GB, 80 of which is the system but it says that that's still 750 GB of storage on my HDD. What's going on?
About Time Machine local snapshots - Apple Support
And to make sure that you have storage space when you need it, snapshots are stored only on disks that have plenty of free space.
750GB sounds like plenty of space for an external hard drive.
Local snapshots are stored on the internal hard drive not the external hard drive. How much free space do you have on the internal hard drive? Also note the below which is the last sentence from the article Mike posted.
"That's why Finder and Get Info windows don't include local snapshots in their calculations of the storage space available on a disk."
Question: Time Machine could not create a local snapshot to back up from because there was insufficient free space on the source volume(s).
