My guess would be there's a dodgy file in the root of that drive. This would be confirmed if you can directly open a subfolder (by typing X:\foldername, where X is the drive letter of the external drive and foldername is a folder you know exists on it, into the start menu) and that DOESN'T crash Explorer. If that's the case you then need to find which file is causing the problem, which could be a bit of a faff!