For some reason timemachine (sometimes) leaves a bunch of local thin snapshots on the disk which causes an error when trying to backup for example iphone to disk (using iTunes). Last time I had about 230G of available disk space (reported by df and About This Mac -> Storage) but iTunes still crapped out with a error message saying that there’s not enough diskspace.
Fix: Open a terminal and run the following command:
tmutil thinlocalsnapshots / 999999999999999 4
Output will look something like this if any local snapshots are removed:
dennis@macscape ~> tmutil thinlocalsnapshots / 999999999999999 4
Thinned local snapshots:
com.apple.TimeMachine.2019-12-05-122627
com.apple.TimeMachine.2019-12-05-111611
com.apple.TimeMachine.2019-12-05-095814
com.apple.TimeMachine.2019-12-05-084121
com.apple.TimeMachine.2019-12-05-073237
com.apple.TimeMachine.2019-12-05-062628
com.apple.TimeMachine.2019-12-05-052302
com.apple.TimeMachine.2019-12-05-042031
com.apple.TimeMachine.2019-12-05-031749
com.apple.TimeMachine.2019-12-05-021517
com.apple.TimeMachine.2019-12-05-011122
com.apple.TimeMachine.2019-12-04-235834
com.apple.TimeMachine.2019-12-04-224844
com.apple.TimeMachine.2019-12-04-214647
com.apple.TimeMachine.2019-12-04-170631
com.apple.TimeMachine.2019-12-04-155048
com.apple.TimeMachine.2019-12-04-142104
com.apple.TimeMachine.2019-12-04-132222