VirtualBox came in handy in the past when a larger hard drive was added to a Series2 TiVo. Once again, it came in handy when installing Windows 10 on a drive already installed in a computer tower alongside Mageia9 and a few additional drives.
There used to be a Win7 installation on /dev/sdb, but it crashed. MS Windows is notoriously known for assuming it is the only OS belonging on a machine and not playing well with others. When Win7 was originally installed on this machine, it was installed on the only drive in the machine, then moved to /dev/sdb and Linux installed on /dev/sda. Of course, all the other drives could have been disconnected and Windows10 installed, but what's the fun in that? The above referenced TiVo procedure provided the inspiration.
In modern versions of VirtualBox, it is possible to install the guest OS to a physical drive. The physical drive in this case is /dev/sdb, so the process is as follows:
$ cd ~/VirtualBox\ VMs
Create the .vmdk file that references the physical drive:
$ VBoxManage internalcommands createrawvmdk --filename win10_sdb.vmdk --rawdisk /dev/sdb
Now, ~/VirtualBox\ VMs/win10_sdb.vmdk references the physical drive, so when doing the creation of the virtual machine, that file will be used as the drive. Configure the optical drive to use either a DVD image or as a passthrough to the physical optical drive for the Win10 install DVD. It's possible to use a USB drive for the installation media, but that option has not been explored by this writer.
Start the virtual machine and install Windows 10. When the first round of installation is completed, stop the virtual machine completely; it's job is done.
Using the BIOS boot election, boot from /dev/sdb and allow Win10 to continue its installation. It will note that the hardware has changed and will install the appropriate drivers.
Boot back into Linux and use the boot configuration process to discover the "foreign OS" and the GRUB2 boot menu will now allow the choice between Linux and Win10.
There are some useful tips for installing a slimmed-down version of Windows 10 to be found in The Sims Complete Collection on WinXP.