Using VirtualBox on Mac HighSierra - installing Windows 10 via external hard drive, not working!
VirtualBox can emulate a DVD (optical) drive. To insert a DVD into an emulated drive, you must first convert the physical DVD to an iso file. You can select this iso file under Settings, as shown below. By the way, if Virtual Box did recognized your external DVD drive, then this external DVD drive could also be selected here. In the image below, my internal DVD drive is listed as Host Drive 'HL-DT-STDVDRW  GA32N'.


  Note: You can click on an image to get a better view.




The instructions below were copied from this Apple website.

Create your own ISO

Use these steps to make an ISO copy of your Windows DVD:


While your Mac is started from OS X (or macOS), insert the Windows installation DVD in an external optical USB drive.
After the installation media mounts in the Finder, open Disk Utility from the Utilities folder (choose Go > Utilities).
Select the USB optical drive so it's highlighted in the Disk Utility window.
Choose File > New Image, and then select your Windows DVD from the submenu.
Choose DVD/CD master from the Image Format menu, then choose none from the Encryption menu.
Give the new file a name, then click Save. If prompted, enter an administrator user name and password. A progress bar appears while the image file is created (it might take some time to create the image file).
After imaging is done, eject your Windows installation disc. Disconnect the external optical drive from the USB port after the disc ejects.
From the Finder, locate the disk image file you created earlier. Single click on it to select it, then press the Enter or Return key on your keyboard to rename the file. Change the disk image file extension from .cdr to .iso. When prompted, click "Use .iso" to verify this change.




Note: Another possibility would be to download the Windows iso from this Microsoft website.