Why can't I mount the Ubuntu 12.04 installer ISOs in Mac OS X?

Over the past few days, I have downloaded both the 32 and 64 bit version of server and desktop to install on an Intel based PC.

It is normal from within OS X to double click on an ISO and it will mount the ISO in the finder as well as within disk utility.

When I attempt to mount any of the Ubuntu ISOs I downloaded, OS X returns the error message The following disk images couldn't be opened. The reason given for the error is no mountable filesystem.

However, I am still able to open the ISO directly from disk utility and burn it to a DVD/CD.

What has changed in this release to cause this? Is there something wrong with the current ISOs?

The OS X machine I am using is only two weeks old and is having no issues with any other ISO.

Solution 1:

This is due to a bug (of sorts) in the package used to create the Ubuntu ISOs

Since 11.10, Ubuntu ISOs have been "hybrid", meaning that the ISO can also be written bit-for-bit to a USB device to make it a working Live-USB, without having to use StartUp Disk Creator, UNetBootin, etc.
- Note that this "hybrid" is different from the OS X/Disk Utility definition of "hybrid", which simply refers to an ISO readable on both Mac (HFS+) and Windows (ISO9660).
Ubuntu's hybrid functionality is provided by the isohybrid script in the SysLinux bootloader. It works by encapsulating the standard ISO9660 CD filesystem within a partition, so that it will work unmodified if written to a USB/hard disk.
This patch/bug report discusses a problem in the way isohybrid sets GPT UUIDs, causing Disk Utility/OS X to fail to recognize the hybrid filesystem and consequently, failing to mount it. Hopefully it will be fixed at some point, making future Ubuntu hybrid ISOs compatible with Disk Utility.

Alternative to extract/explore Ubuntu ISOs on OS X: Keka

Keka is an open-source OS X frontend to the p7zip package, which is able to view and extract ISOs in addition to a myriad of other archive management features. While it won't let you "mount" the Ubuntu ISO on its own, it should let you view the ISO filesystem and extract what you need.

Solution 2:

In case anyone else comes here because they were trying to install ubuntu in Vmware Fusion, but couldn't mount the iso, at least in Vmware Fusion 4, you can "Continue without disc" -- then it'll let you choose an iso on the next screen, so OSX never needs to mount it, and Vmware will mount it just fine.

Solution 3:

Maybe because they are now hybrid (DVD/USB) ISOs which can be directly dd'ed to a USB drive and booted.¹

^{1. https://lists.ubuntu.com/archives/ubuntu-devel/2011-June/033495.html}