technically yes, as long as your BIOS supports booting from a usb device (most newer do) then it is possible. Although, you really don't want to do that because you will be severely limiting your system, because it can only read/write as fast as the usb 2.0 can go
eSata would be a much better idea since you can get power and data connectors from the computer instead of having IDE and molex connectors hanging out (unless the power is connected externally and already booted up - but you would still have the IDE hanging out)