virtualbox.org

I have Vbox 5.0.24_Ubuntu r108355 running on Linux Mint 18

My D drive is now "lost". Attempting to access it gives a message:
"D:\ Application not found."

It used to work: I installed MS Office, other s/w with no problems.
I have tried several Windows install disks. Even a data disk.

Help, please!

First, that version means that you're running the Ubuntu VirtualBox fork. If you want support here then you'll need to install actual VirtualBox, not a fork.

Also, your question is actually a Windows support question, and we don't normally deal with those around here. However, I'll briefly point out that the error says that its missing something from the D drive, not that the D drive itself is missing. The latter would be perfectly possible if you deleted the drive from the VM recipe... but that isn't what the error message says.

It looks to me like you have a shortcut to an application on the virtual CD drive, and now you have a different CD in the drive, therefore it fails to find the path. Going for a total guess, I'd say that you created then shortcut, and then you installed the Guest Additions, which involves changing the CD.

It is not normal to run software directly from a CD drive. Typically, PC software will have a CD installer. If it doesn't have an installer on the CD then you just create a folder on your virtual hard drive and copy the CD contents there. Then you create your shortcut to point to the app in the hdd folder.