FastDMG is a free, open-source Mac utility that mounts disk image files — .dmg, .iso, .img, and other image formats — dramatically faster than the system default, skipping the verification step that causes macOS to stall before opening anything.
What is FastDMG?
FastDMG is a lightweight replacement for macOS's built-in disk image mounting mechanism, written by Sveinbjörn Þórðarson (the same developer behind Platypus and Sloth). It registers itself as the default handler for disk image file types and opens them immediately — no spinning progress bar, no "verifying" interstitial that can take ten or more seconds on a large installer.
The difference is visceral the first time you double-click a downloaded .dmg and it mounts in under a second. If you install a lot of software, update apps manually, or work with disk images in any professional capacity, that cumulative saving compounds fast.
What does FastDMG do best?
FastDMG's core superpower is instant mounting — it calls hdiutil directly and bypasses the checksum-verification pass that Apple's DiskImageMounter runs by default. For the vast majority of downloads from trusted sources, that verification adds seconds (occasionally much more) for no practical benefit.
- Broad format support: handles .dmg, .iso, .img, .cdr, .toast, and other hdiutil-compatible formats, so it works as a catch-all image launcher, not just an app-installer helper.
- Zero interface: there is no window, no menu bar item, no preferences pane — FastDMG simply does its job and gets out of the way.
- Open source: the full source is on GitHub, so you can audit exactly what it does before handing it file-open duties on your system.
- Tiny footprint: the app bundle is a few hundred kilobytes; it adds nothing to login items and consumes no background CPU.
I have used FastDMG as my default .dmg handler for months. The only time I notice it is when I temporarily switch to another Mac that does not have it installed — suddenly every .dmg feels like it is mocking me with a progress bar.
Is FastDMG free?
Yes — FastDMG is completely free to download and use, with no in-app purchases, no nag screens, and no licence key. It is released as open-source software by its author. If you want to support development, the project page links to a donation option, but it is entirely optional.
Who should use FastDMG?
FastDMG is ideal for anyone who regularly installs or tests Mac software distributed as disk images. Developers who download nightly builds, beta testers, IT administrators imaging machines, and power users who prefer to manage their own app updates outside the Mac App Store will feel the most benefit.
If you only mount a .dmg once a month, the default system handler is probably fine. But if your Downloads folder fills with disk images the way mine does, FastDMG is one of those small tools that punches far above its file size.
It is worth noting what FastDMG is not: it is not a disk manager, not a partition tool, and not a replacement for Disk Utility. It handles one thing — opening image files quickly — and does not attempt anything beyond that scope.
What are the best FastDMG alternatives?
The most direct alternative is macOS's own DiskImageMounter, which comes pre-installed. It is reliable and integrates with Gatekeeper verification, but that verification is precisely the delay FastDMG eliminates. For users who want a GUI around disk images with additional management features, Disk Diag or the fuller Disk Utility cover more ground at the cost of more complexity. Some developers use hdiutil attach directly in the Terminal for scripted workflows, which achieves the same speed but requires leaving the Finder. FastDMG occupies the sweet spot: hdiutil's raw speed wrapped in a proper file-type handler that integrates invisibly with the OS.
How does FastDMG compare to macOS DiskImageMounter?
The practical difference is speed and silence. DiskImageMounter displays a verification progress sheet before mounting — a safety check that reads the entire image to validate its checksum. FastDMG skips that step and calls through to mount immediately, which means images that took eight to fifteen seconds to open now open in under one. Both tools ultimately rely on hdiutil under the hood, so the mounted volume behaves identically once it appears on the desktop. The trade-off is clear: FastDMG prioritises speed; DiskImageMounter prioritises verification. For software you download from known, trusted sites, the verification step is largely redundant — your browser and Gatekeeper have already done the heavy lifting.
