One of the most annoying aspects of using VirtualBox is having to change the type from "Normal" to either "Immutable" or "Multiattach" after adding an existing disk to a VM using the GUI. I sometimes share VDI images between systems, as installing an OS 5 times can be a real pain, so I just keep a nice collection of pre-installed VirtualBox working VDI images on an external drive.
Currently the process for changing a just added disk to say "Multiattach" for the purpose of testing the same OS in multiple scenarios is as follows:
Create VM, and attach an existing disk copied from another source.
Go into the Media Manager within VirtualBox GUI, update the "Type", which releases the disk from the just created VM.
Open the VM settings again, reattach the VDI which now involves looking through your file system for the VDI.
It would be really nice if the type was asked when attaching a new disk image to your VM, with the default being "Normal" of course. An added bonus would be if you add a VDI from a read-only source, like a read-only network share, or a DVD-ROM(I did this back in the day to save HDD space), it would default to "Immutable", and provide the other sane option of "Multiattach". Read-only VDIs are also common on shared workstations, where a bunch of disk images is stored in a common read-only local directory, where multiple users of that same machine have access to.
The idea behind using "Immutable" disk images is obvious when you think of software testing for developers on multiple target OSes. You need to keep a very consistent test environment, and immutable images are absolutely perfect for this.
Yes, I do realize that VBoxManage storageattach supports the "--mtype" parameter to update this upon attaching the disk to a machine, so this can all be done programmatically. However, from a ease of use point of view, and thinking of a general consumer who may never use VBoxManage, having this option directly in the GUI will be very nice. Even a way to enable or disable such a dialog box in the preferences will go a long way.