virtualbox.org

I'm working on a Windows 8.1 host @ Surface Pro 3. Host system and VBox are both up to date.
The guest system I was trying to install was Windows XP Pro SP3, from an MSDN image. Haven't checked its hash value, but I doubt I'll need that.

The image was on a portable USB hard drive. During installation, the program reported file missing after asking me for localization, password and so on. And I found the hard drive was somehow unmounted from my host system. No single notification from the host system or VBox.

This should not be intended right? How may I help to provide further information?

VirtualBox has no mechanism that I know of for unmounting a USB drive from the host.

Unless, if one were unwise enough to define a blank USB filter in the VM then that might certainly do it. A blank USB filter will basically just grab the first USB device it sees at random, stealing it from the host and assigning for the exclusive use of the guest. Likewise if you created a filter which specifically selected that drive: but surely you would know if you did that, and wouldn't be here asking the question!

Basically, don't tell VirtualBox to assign a drive exclusively to the guest if you want the host to continue using it.

How very curious... I checked my USB setting for that VM and no USB filter was there. I don't suppose simply enabling USB controller would produce the problem?

Basically I just tried install the guest system immediately after creating a brand new VM. I actually don't know anything about "assigning a drive exclusively to the guest".

No, the problem can only be provoked by stealing said device from the host, via a USB filter, or manually via the VMs Device menu.

If it isn't that then your problem has nothing to do with VirtualBox. As I said, I has no other mechanism that might unmount a host drive.