Windows 7 does not boot when its hard drive is connected externally

[wolfs]

Hello,

I removed my hard drive containing Windows 7 from my computer, where it boots properly. I placed the drive in an enclosure and connected it via USB. It refused to boot, instead flashing a blue screen for less than a second and then asking if I wanted to attempt system repair.

Upon returning the hard drive to the inside of the computer it works correctly as before.

I regularly boot various Linux operating systems with hard drives connected internally or externally on this computer, so I believe that my BIOS settings are correct and that there is not a problem with the hardware.

Does anyone know why Windows refuses to boot unless it is connected internally?

Thank you for any help or advice.

 

[SIW2]

Windows doesn't boot via usb natively. It is possible to fiddle with it to get it to work. Not sure of the point.