To remove/unmount this drive, right click on its drive letter icon and pick Eject menu command
What about older windows? The API infrastructure exists on windows 7 too, but it is only accessible through the disk management applet, so it isn't as convenient to mount and unmount VHD files. Instead of running DISKMGMT.MSC, I found a clever solution that adds the mount commands to send to context menu for easier access. In essense they use diskpart command line tool to select to attach the VHD as such:
SELECT VDISK FILE="c:\path\test.VHD" ATTACH VDISK
There is no easy way to automatically choose the drive letter, diskpart picks usually the first available drive letter, but you can manually change it if required.
When it comes to ISO image files, there is no system way to mount them before windows 8. But there are plenty of 3rd party tools, or you can use xplorer² own compressed archive shell extension that among other things can browse into ISO files — without even mounting them first on a drive letter.
That's all folks!