To select a drive you simply have to use the drop down box on the top left-hand side. All the devices found during the system scan are listed there.
To open an image file, click on the Open Image File button in the toolbar.
Once an image file has been opened, a virtual device will be added to the drop down box on the top left-hand side. The virtual device stays active and the image file stays loaded even when you switch to another device in the drop down list. Only if the image is closed will the virtual device be removed again. You can open an infinite number (system resources limited) of image files at the same time. Each time you open an image file a new virtual device will be added to the drop down list. You can then switch between image files by just selecting the preferred one via the drop down box.
The "Open Recent" option allows you to quickly open previously opened image files, if they are still located at their previous location. You can remove files which are not present anymore on their previous location by "Remove Obsolete". "Remove All" cleans the entire list. The list acts as a FIFO: First In, First Out.