USB HDD (win7) to 2 PC's in dual boot

It is straight-forward to run linux from a USB drive. But Windows does not boot from USB. I think it is because Microsoft limits the usage of each system to one computer.

A workaround could be to install an Ubuntu flavour (in a USB HDD), install a virtual machine (for example VirtualBox), and install Windows in the virtual machine.

There are many USB/Sata interfaces/boxes that work well. There are also interfaces that can also coect via eSATA, which is much faster than USB 2.

I have a box from StarTech 'USB3.0/eSATA Trayless 3.5" HDD Enclosure with UASP', but there are many other alteatives. If you need only USB (not eSATA), you can find many less expensive alteatives.