I noticed that when I emptied the trash in High Sierra, I didn't see a corresponding rise in available space in the status bar of open Finder windows. I opened "About This Mac" to the "Storage" tab. Here I could see that my System was using 70GB. <img src="{att_link}huge_system.png" href="{att_link}huge_system.png" align="none" caption="Huge system (not mine)" scale="50%"> A search turned up the article <a href="https://discussions.apple.com/thread/8084507" source="Apple Support">macOS High Sierra – Huge System Storage</a>, which explains that High Sierra introduced a feature called local backups for Time Machine (<a href="https://support.apple.com/en-us/HT204015" source="Apple">About Time Machine local snapshots</a>). It does pretty much what you would expect it to do: <bq>Your Time Machine backup disk might not always be available, so Time Machine also stores some of its backups on your built-in startup drive and other local drives. These backups are called local snapshots. One snapshot is saved every hour. If you're using macOS High Sierra, another snapshot is saved before installing any macOS update.</bq> And Apple swears that this feature won't get in your way: <bq>Snapshots older than 24 hours are automatically deleted. 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. When storage space gets low, additional snapshots are deleted, starting with the oldest. <b>That's why Finder and Get Info windows don't include local snapshots in their calculations of the storage space available on a disk.</b> (Emphasis added.)</bq> I've found that the local backups <i>area</i> reflected in the numbers shown in Finder windows. When I deleted local backups, one of them caused over 40GB to be recovered. But today, I found that the local backups had continued, but weren't taking that much space. I think this feature just uses more apparent space when you've recently done a large system update (as MacOS has had of late). I'm not convinced that these backups are truly removed after 24 hours, though. For even more space back, you can delete older iOS backups to get back GBs of space. <ol> Open iTunes Open Preferences Select Devices (second-to-last icon) Delete old backups </ol> I had a good dozen gigabytes of backups from 4 years ago that I could delete.