I dont have an external drive, but should realy invest in one. I never heard of your problem before with external or internal drives. Just to rule out some basic stuff, have your checked your machine for any malware, virus's, trojans and the like. Checked for any file and registry corruptions and cleaned out all temp files that could be causing your issue. Where the two external drives of the same make? If so, could check out external HDD manufacturer website for any known issues of that type. You could hook up problemmatic hard drive on a friends computer and see if it duplicates the problem on that PC. If it does, chances are the problem is with the HDD itself.