Unable to mount .dmg files in OS X - "Device not configured" error. How to fix it.

I had downloaded some DMG files to install some software and kept
getting these error messages:

Unable to attach "Foo.dmg" - Device not configured.

It was happening for more than one .dmg file, and I'm sure the
download was not corrupted.

At first, I tried using the Disk Utility, thinking it was related to
disk permissions, but the option wasn't present when I selected the
DMG. Clicking on "Verify disk" didn't help either, and resulted in the
same error "Device not configured".

Tried the instructions below and they worked.

1. Go into terminal

2. type

su [username that has admin rights]

you will be asked for your admin password.

3. Then run these commands. (I don't know what they do but they worked)

sudo kextunload -c com.apple.AppleDiskImageController

sudo kextload -b com.apple.AppleDiskImageController

4. Now try to mount those .dmg files.

Addendum: It's still not fixed. Now I'm just getting a new error message - "no mountable file systems". Oh great.