Is this right; your theory is install OpenBVE on an external hard drive so you'll never have an internal hard drive failure again?
Along that same line of logic, if I don't drive my car and drive my neighbor's car instead, my car will never get into an accident, will suffer much less wear, will last much longer, and maintenance will be near $0. Just wish my neighbor would agree to this arrangement.
Sounds like a more serious problem with your PC hardware &/or operating system. I've got three PCs, ages 3 years to 15 years, with 7 hard drives amoungst them; never had a hard drive failure one. I do run scheduled maintenance (defrag, antivirus, crap cleaner, reboot etc) each night.
An external HD will give you the ability to store/backup critical files and conveniently move them to another PC, if your internal HD fails again.