DropDMG supports the following disk image and archive formats:
The contents of a read-only disk image can be directly accessed from the Finder, but they cannot be modified. Read-only disk images support licenses. They have internal checksums for use with the Verify Image… command. In order to search a read-only disk image via Spotlight, use the Mount Image… command and select the Shadow file option.
The contents of a read-write disk image can be directly accessed from the Finder, and you can add, remove, and modify files. Read-write disk images can be created from existing files/folders or by using the New Blank Disk Image… command.
The disk image is stored as a single file. Compatible with macOS 10.2 and later.
When using the macOS Extended file system, this is less efficient (in terms of space used) than a sparse disk image because the file size is determined by the capacity of the disk image rather than by the amount of data currently stored on it. This can be desirable, for example, if you want to reserve a certain amount of space for the disk image’s use.
When using the APFS file system, this is more efficient (in terms of space used) because macOS will use APFS’s native support for sparse files to automatically reclaim unused space, without your having to manually compact the disk image. However, care must be taken to keep the .dmg file sparse; many common ways of moving and copy such files will make them balloon to full size. Lastly, writable .dmg files are slower to write than .sparsebundle files.
Archives make your files accessible to computers running other operating systems (Windows, Linux, etc.). Archives are slower and require more disk space than disk images, as they must first be expanded before the contents are accessible in the Finder. DropDMG creates special archives that preserve resource forks and Mac metadata. This extra information is ignored by other operating systems.
NDIF disk images are compatible with System 7, Mac OS 8–9, and macOS. These can be useful for exchanging files with legacy Macs or for use with emulators such as Sheepshaver and Basilisk. NDIF disk image files use resource forks, so some care must be taken in transferring them over the Internet. They are also limited to 2 GB of logical size. macOS 11 and later do not support creating NDIF disk images.
Note: Microsoft Windows also uses disk image files whose names end with .img, but it does not read the NDIF format. To create a .img disk image for Windows, tell DropDMG to use the .iso or .cdr format and then rename the file to .img.