Can a USB storage device be the primary (and only) means of storage?
No. USB-based mass storage devices cannot be the primary hard disk storage solution on a regular system (Microsoft Windows Logo Program System and Device Requirements, B10.1.5.6). However, these devices might be expected to be a replacement for booting to load an operating system (for example using a CD-ROM drive over USB) on the primary boot drive or as a replacement for legacy floppy disk drives. Booting from an external USB CD-ROM or USB floppy disk drive requires BIOS support.