How macOS actually works
Half the software on a Mac arrives as a .dmg, and the file is widely misunderstood: people run apps from inside it for months, or hoard gigabytes of them out of caution. Both have simple fixes.
A .dmg is a file that behaves like a removable drive. Double-click it and macOS *mounts* it — a volume appears in Finder and on the desktop, exactly as if you had plugged in a USB stick containing the app.
The install ritual — drag the app to the Applications shortcut — is a plain copy from that temporary volume to your disk. Nothing more.
Running the app from inside the image. It works, which is the trap. The app lives on a read-only mounted volume; it cannot update itself properly, and it vanishes from the Dock's reality when the image is ejected. If an app disappears whenever you restart, this is why. Fix: copy it to /Applications, eject the image, launch from Applications.
Keeping every DMG forever. After the copy, the image is packaging. The app does not need it, and re-downloading is the real backup. A Downloads folder audit usually finds a surprising total:
find ~/Downloads -name "*.dmg" -exec du -h {} \; | sort -hrA mounted image locks its file. If deleting a DMG fails with "in use", eject the volume first — the eject symbol next to it in Finder's sidebar, or drag the desktop volume to the Trash.
Software you paid for whose vendor might vanish, or old versions you may need to reinstall — keep those DMGs, on an archive drive, named by app and version. For everything current, the vendor's download page is the archive.
Yes. The DMG is packaging; the installed app in /Applications is complete without it. Keep only installers you could not download again.
The image is still mounted as a volume. Eject it in Finder's sidebar first, then delete the file.
Everywhere keeps track of every file on your Mac — including the folders Spotlight hides and drives you have unplugged. Free for a day, then $19 once.
Download for MacVersion 2.15.7 · macOS 13+ · 3.8 MB · notarised