Macos – Can’t unmount dmg. Keep getting “Resource Busy”

I have just had the same problem, with a disk image created by Disk Utility itself. In my case, the disk image is whole disk image, i.e. it contains a bare filesystem, no partition map whatsoever. Neither Disk Utility nor hdiutil attach disk.dmg would work, both complaining about the dreaded "no mountable file systems" issue.

What helped was to mount it from the command line, in two easy steps:

hdiutil attach -noverify -nomount disk.dmg

This only creates the device node. Now run

diskutil list

and note the diskN part of your image, where N is a positive integer.

Finally, to mount the device, do

diskutil mountDisk /dev/diskN

where diskN is the device identifier you determined before.

Note that you are mounting the entire disk, whereas normally you would mount a slice, such as disk0s2.

Be aware that this helps if your problem is with a disk image similar to mine. Nonetheless, it should still work with partitioned disk images, see man diskutil.

Disk images are just containers that emulate a disk. The DMG's contents are distinct from the DMG container. So you've probably only converted the container to read/write.

For example:

We can convert a DMG that contains an ISO to be read/write, but the ISO itself can only be read-only:

 ___________________        ___________________
|                   |      |                   |
|  Disk Image (r/o) |      |  Disk Image (r/w) |
|  _______________  |      |  _______________  |
| |               | |  ==> | |               | |
| | ISO9660 (r/o) | |      | | ISO9660 (r/o) | |
| |_______________| |      | |_______________| |
|___________________|      |___________________|

You run into a similar problem with the hybrid filesystem images that many OS distributions are shipping these days.

Here's an excerpt from the hdiutil(1) man page section on Hybrid images:

The generated image can later be burned using burn, or converted to another read-only format with convert.

The generated filesystem is not intended for conversion to read-write, but can safely have its files copied to a read/write filesystem by ditto(8) or asr(8) (in file-copy mode).

So there's the work-around: Copy the files off and make another DMG.

Which, unfortunately, is probably what you were hoping to avoid.

By the way, you might find this command helpful to peek into the DMG's partitions:

hdiutil pmap your_file.dmg