It could be a voltage issue. It could be a motherboard issue. Or it could be a the drive. For me, step one is usually parts swapping. You have a working internal drive, so you could swap them. But that I mean open both cases, remove the external floppy cable and insert it into the internal. This way you dont even have to remove the internal drive, its just hooked up to the external port. If it works, you can rule out any motherboard issue/connector/cable/etc. If it does not, you can focus on other issues.