How to back up a Mac that has no space left to back up with
Backups need working room, which is exactly what a full Mac lacks. The order that gets you backed up without deleting anything first.
The uncomfortable position: the disk is full, you want to clear it, and you should not delete anything without a backup, but the backup itself needs room to work. There is an order that resolves this without gambling.
Why a backup needs free space at all
Time Machine takes a local snapshot before backing up, writes temporary files, and needs room for the index it maintains. A completely full disk can therefore fail to start a backup at all, which is the failure people hit first.
The order that works
- Delete only what is unambiguously disposable. The Trash, installers in Downloads, and browser caches. None of this needs a backup because none of it is yours.
- Thin local snapshots.
sudo tmutil thinlocalsnapshots / 20000000000 4asks for roughly 20 GB. Snapshots are not a backup of anything you are about to lose. - Back up the irreplaceable things first, by hand. Documents, Photos, Desktop and any project folder, copied to an external drive. This is not a full backup and it is the part that matters.
- Then run a full backup, which now has room to work.
- Then clean up properly, with everything safe.
df -h /System/Volumes/Data
tmutil listlocalsnapshots /
sudo tmutil thinlocalsnapshots / 20000000000 4Excluding what does not need backing up
In Time Machine settings, excluding virtual machine images, downloaded media and build output makes the first backup dramatically smaller and faster. Everything excluded is either replaceable or reconstructible, which is the same distinction used throughout the order to free 100 GB.
| Exclude | Why |
|---|---|
| Virtual machine images | Huge, change constantly, rebuildable |
| Docker's disk | Same, and prunable instead |
| node_modules and build output | Regenerated by a build |
| Downloads | Almost all of it is installers |
| Cache folders | By definition disposable |
If there is no external drive at hand
Copy the irreplaceable folders to cloud storage, which needs no local space, or to another Mac over the network. It is slower and it is a real backup of the things that matter, which is enough to make the cleanup safe.
Once you have room again, the ordinary sequence applies, and why the disk filled up in the first place is worth reading so it does not recur.
Common questions
How do I back up a Mac with no free space?
Delete only unambiguously disposable things first, thin local snapshots, copy irreplaceable folders to an external drive by hand, then run the full backup once there is room to work.
Why does Time Machine fail when the disk is full?
Because it takes a local snapshot and writes temporary files before backing up, both of which need free space on the Mac itself, not only on the destination.
What should I exclude from a Time Machine backup?
Virtual machine images, Docker's disk, build output such as node_modules, Downloads and cache folders. All are replaceable and all make the backup dramatically larger and slower.
Can I clean up before backing up?
Only things that are clearly disposable: the Trash, installers, and caches. Anything you would miss should be backed up first, which is why copying the irreplaceable folders by hand comes before the full backup.