I’m backing up my 2 TB internal SSD to an external 2 TB SSD drive. Time Machine estimates the backup to be 1.13 TB, so it should easily fit into the external drive.

Backup estimate

However, even the first backup doesn’t complete because the backup disk is full before the first backup finishes (only 1.3 GB left):

Disk Utility

Finder is not showing any backups:

No backups shown

I tried formatting the external disk, but the issue keeps occurring.

How can I find out what’s causing this?

OS version is macOS 11.6 (20G165).

  • 1
    Do you have an external drive connected? If yes - make sure you add it to the "Excluded" list. Oct 15, 2021 at 13:36
  • No, the backup disk is the only external drive.
    – aaronk6
    Oct 15, 2021 at 13:37
  • Maybe the answer here or some of the discussions might help in case you have not seen it: Time Machine thinks the size of my backup is bigger than the source
    – Alper
    Oct 15, 2021 at 14:41
  • 1
    Do you have a Dropbox, OneDrive, or GoogleDrive volume on this machine configured for offline access? Time Machine isn't going to know how that works, and the client(s) are going to try to sync the local data as TM reads the files. Exclude such directories. Oct 15, 2021 at 17:44
  • 1
    @MarcWilson Yes, I actually have both Google Drive and Dropbox installed and configured for streaming. I’ll try excluding those directories and report back!
    – aaronk6
    Oct 17, 2021 at 9:32

1 Answer 1


Check to see if you have a Dropbox, OneDrive, or GoogleDrive volume on this machine configured for offline/streaming access.

When Time Machine reaches those directories, it will read the files, and the client (Dropbox, Google Drive, whiatever) will respond by streaming the data, which Time Machine will write to the backup as though they were regular files.

If the on-line storage is large enough, Time Machine will overflow the backup medium while streaming the files.

Exclude such directories.

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