Mister Plimsoll

Mister Plimsoll watches your Mac's drives and warns you before any of them gets too full. Set a percentage threshold for each volume you want to monitor—your startup disk, external SSDs, or Time Machine destinations—and you'll get an alert when usage crosses the threshold. This helps you take action before a volume becomes too full, which can lead to performance or other problems.

You can choose to receive an alert in one or more of the following ways: macOS notification, floating window, email (you pick the address), Pushover push, or webhook to Slack, Discord, IFTTT, or Zapier. Turn on as many or as few as suit you. You can set the maximum number of notifications per day per volume. 

A critical-override threshold (off by default) will keep notifying you after the daily cap is reached when volume is genuinely near full capacity. Mister Plimsoll lives in the menu bar (Dock icon optional), collects no analytics, and stores all settings on your Mac.

Because Time Machine snapshots can sometimes fill startup volumes, once the threshold is reached on regular volumes, Mister Plimsoll shows how much storage is occupied by those snapshots. On Time Machine destinations, the app always shows the storage used for regular Time Machine backups.