Usb type to reflash pureboot?

Hi, I want to reflash the latest pureboot (20) onto a mini V2 (with Qubes os).
Can I use a mass storage SATA usb device with multiple files on it including the pureboot .rom?
I mean will Pureboot give me the option to choose from the many files on the drive or does it need to be the only file on a usb memory stick to work?

See also: