Software Recommendation – How to Free Up Disk Space

Both sparse disk images and the newer sparse bundle disk images intentionality do not reap/reclaim space until they need to. This is especially so for a sparse bundle - which is designed to reduce the network transfers when the image is stored on a network share. It certainly speeds up normal read/write operations since the system can assume compact won't happen when the files are changing. When you check things in disk utility, it perhaps sees that space could be reclaimed and does that for you. You are correct, that in normal use, it ends up with you using more space than if you simply put things on the normal file system.

You could convert your storage to a normal sparse image to get more traditional freeing of space when you delete files stored within the disk image.

Certainly read the first two paragraphs of the man page for hdiutil.

You can of course kick off a manual compact operation with:

• hdiutil compact /path/to/sparsebundle.dmg

There certainly isn't the same amount necessary as the DMG, so you sure can mount a 100GB DMG into a file system that has less (there will be trivial consumptions, maybe a few kb, but nothting that you'll notice).

The whole thing works in the way, that the system attaches a file-system driver to the file with the DMG. The driver will present the DMG file to the system like an "external" drive and perform file requests on behalf of the system on the contents of the file.

Essentially the driver will says to MacOS: We have a new volume here. So MacOS will then ask the driver. "Give me a list of content in it", and then the driver examines the inside of the file and presents the list of files organized inside the DMG. Etc.