VirtualBox Windows 10 64 bit question

[Phil]

I'm having a problem installing Windows 10 into VirtualBox from a 
bootable USB stick.

After a bit of fiddling the bootable USB stick was eventually detected 
and the installations started which made me think it was actually going 
to work. However, after 3 hours of "thank you for your patience" I gave 
up and deleted Windows 10 with the idea of starting afresh.

Normally, I press f12 and select the bootable USB stick to install 
Ubuntu, for example. F12 does pop up inside VirtualBox but the USB stick 
is not included in the list. Where to from here? At 4.7GB a DVD is too 
small to hold the iso file.

The ThinkPad T-420 doesn't have BIOS boot order setting, only a 
networking wake-up boot order.

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Regards,
Phil