I don't recommend you use external for Windows OS period, encrypting it is just further making it a corruption disaster waiting to happen.
External drives are for storage, not for operating systems.
If a external which is a hard drive, a plastic shell, a sata to usb adapter, and a power supply is cheaper then an internal hard drive, then clearly the external must not be as good.
Also just to be clear for others that come across this thread:
External Drive does not automatically equal a backup. If you have the file on both your computer AND the external drive then it is a backup, if you put the only version of a file only on the external drive then it is not a backup (and actually worse then just leaving it on the PC).