Previous Next  Up  Table of Contents  DropDMG Home

5.1.1   Format

DropDMG supports the following disk image and archive formats:

Read-only Disk Image

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.

.dmg — smaller file, faster

A disk image compressed using zlib. The file is relatively compact and quick to access. This is DropDMG’s recommended format for general use. DropDMG creates smaller zlib-compressed disk images than Disk Utility. You can control the level of compression via the ZlibLevel esoteric preference. Compatible with Mac OS X 10.2 and later.

.dmg — smallest file, slower
A disk image compressed using bzip2. Depending on the type of data stored, this can be significantly smaller than a zlib-compressed disk image. It is also slower to create and access. Compatible with Mac OS X 10.4 and later.
.dmg — no compression
An uncompressed disk image will always be larger than a compressed one. However, with certain types of content (music, photos, already compressed files) the difference in size will be slight, and an uncompressed disk image will generally be faster to create and access. Compatible with Mac OS X 10.2 and later.
Read-write Disk Image

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.

.sparsebundle — most efficient
Sparse bundle disk images appear as a single file but are actually stored as a folder with many files inside. This makes it more efficient to back them up using Time Machine or other backup utilities, as only the changed parts need to be copied. Additionally, sparse bundle disk images work well with the Compact Image… command. Compatible with Mac OS X 10.5 and later.
.sparseimage — more efficient
Sparse disk images are stored as a single file. This can make backups inefficient for large disk images, because the entire file must be copied each time. The Compact Image… command generally does not reclaim as much disk space as with sparse bundle disk images. Compatible with Mac OS X 10.3 and later.
.dmg — constant file size
The disk image is stored as a single file. This is less efficient 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. Compatible with Mac OS X 10.2 and later.
Cross-platform Archive

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.

.tar.gz — smaller file, faster
A tape archive, compressed using gzip. The file is relatively compact and quick to create and expand. Compatible with Mac OS X 10.4 and later.
.tar.bz2 — smaller file, faster
A tape archive, compressed using bzip2. Depending on the type of data stored, this can be significantly smaller than a .tar.gz archive. It is also slower to create and expand. Compatible with Mac OS X 10.4 and later.
.tar — no compression
An uncompressed archive will always be larger than a compressed one. However, with certain types of content (music, photos, already compressed files) the difference in size will be slight, and an uncompressed archive will generally be faster to create and expand. Compatible with Mac OS X 10.4 and later.
.zip — smaller file, faster
A ZIP archive is similar to a .tar.gz archive except that this format is more common on Windows. DropDMG creates smaller ZIP archives than the Finder. ZIP archives do not preserve creation dates or Unix ownership. Compatible with Mac OS X 10.3 and later.
Cross-platform Disk Image
.cdr DVD/CD-R Master
A master disk image is an uncompressed, read-only disk image. If created from a file or folder, it will have a Mac filesystem; if created from a volume, the filesystem will be unchanged. This is mainly useful for imaging an existing volume (e.g. copying a CD, DVD, or Blu-ray disc) or creating a Mac disk image to burn using a non-Mac computer. Compatible with Mac OS X 10.0 and later.
.iso Hybrid (HFS+, ISO 9660, Joliet, UDF)
A hybrid disk image is like a master. Additionally, the image contains multiple filesystems so that the contents may be accessed using any operating system. Mac-specific files such as .DS_Store are hidden from the non-Mac filesystems. Compatible with Mac OS X 10.0 and later.
Legacy NDIF Disk Image

NDIF disk images are compatible with System 7, Mac OS 8–9, and Mac OS X. 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.

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.

.img NDIF-compressed
A compressed, read-only disk image.
.img NDIF read-only
An uncompressed, read-only disk image.
.img NDIF read-write
A disk image whose contents can be modified.
Previous Next  Up  Table of Contents  DropDMG Home