zlib creates relatively compact disk images that are compatible
with Mac OS X 10.1 and later. This is DropDMG’s recommended format
for general use.
.dmg bzip2-compressed
bzip2-compressed images only work on Mac OS X 10.4 and later, but
they can be significantly smaller than zlib-compressed images. They
are much slower to create and access than zlib-compressed images.
.dmg ADC-compressed
ADC-compressed images are a bit larger and slower than
zlib-compressed images, but they work on all versions of Mac OS X.
.dmg Read-only
Read-only images require Mac OS X and are larger than compressed
images. However, when post-compressed with BZip2 or GZip they may
end up smaller than the other formats. Read-only images are also a
good choice when the folder you are imaging contains internally
compressed files such as MP3s or JPEGs; in this case, using a
compressed image format takes significantly longer and does not
save much more space.
.dmg Read-write
Read-write images can be mounted in the Finder and their contents
modified. They do not support license agreements. DropDMG creates
read-write images slightly larger than needed to hold all the
files. This lets you make minor modifications such as repositioning
icons and creating aliases to other files on the image. Once the
image is exactly the way you want it, you can use DropDMG to
convert the read-write image into a compressed one. Another use for
read-write images is as a secure workspace. If you turn on
encryption, you can read and edit the files on the image as normal.
However, when you unmount the image, no one will be able to access
its contents without the passphrase.
.img NDIF-compressed
NDIF images work with Disk Copy 6.x on Mac OS 7, 8, and 9, as well
as on Mac OS X. They have resource forks and so must be encoded
with a utility such as StuffIt, MacBinary, or BinHex before they
are shared on the Internet. NDIF images do not support encryption,
and they are limited to 2 GB of logical size.
.img NDIF read-only
This is an uncompressed NDIF image whose contents cannot be modified.
.img NDIF read-write
This is an uncompressed NDIF image whose contents can be modified.
Read-write images do not support license agreements.
.smi NDIF-compressed
Self-mounting NDIF images (.smi files) may be used on Mac OS 7,
8, and 9 systems that do not have Disk Copy installed.
Additionally, self-mounting images are the only type that can
display license agreements on Mac OS 9.
.sitx StuffIt X Archive
StuffIt X generally produces the smallest (most tightly compressed)
files, but it is slower than the other formats and StuffIt Expander
is required to to expand the archives. StuffIt Expander is bundled
with Mac OS X 10.0–10.3, but not with 10.4.
.tar Tape Archive
Tar is a common Unix format. It is an uncompressed format, so for
the smallest file sizes you should choose either the GZip or BZip2
encoding option to create a compressed .tar.gz or .tar.bz2
file. DropDMG only supports Tar archives on Mac OS X 10.4 and
later, because earlier versions of the OS cannot create (or expand)
Tar archives that preserve Mac resource forks and other metadata.
To expand a Tar archive, simply double-click it in the Mac OS X
10.4 Finder.
.zip Zip Archive
Support for the Zip archive format is built into Mac OS X 10.3 and
later. It is also compatible with unzipping utilities on Windows.
This format preserves resource forks and Mac metadata when expanded
by the Finder. Do not expand the files using StuffIt Expander,
because it does not preserve the Unix permissions and ownership.
Zip archives do not support encryption or segments.
.cdr DVD/CD-R Master
Master images are uncompressed, so they are not good for archival.
They are useful, however, for storing and transferring the contents
of CD or DVD and can be read by both Mac and Windows disc-burning
software.
Note: Apple made some changes to the disk imaging engine in Mac OS X
10.4.7. As as result, .dmg images created with 10.4.7 and later are
not compatible with Mac OS X 10.0 or 10.1.