David, what kind of external hard drive are you using? How is it formatted, what file system? If NTFS, then how are the permissions set on the hard drive?
I am asking these questions since the behavior you are reporting is certainly not normal. For example, I have an external hard drive which I can connect to 7, Vista or XP with exactly same effect.
Just to make things clear - it does not (or at least should not) matter, how you put the files on that hard drive (from Vista, XP, or OSX for that matter). As long as the file system format is compatible with Windows 7 (i.e. FAT32 or NTFS mostly) the hard drive should be recognized and you should be able to read/copy/manage the files on it, unless the permissions on the NTFS file system are set in a wrong way.
So, my advice is as follows: connect your hard drive to Windows 7, make sure that the OS recognizes the drive and that there is no conflict of any kind (check the device manager, make sure the driver
is in order and there is no yellow "unknown device" icon there); check that the drive appears properly in Explorer and the right-click on it, go to security properties and make sure you (as the current user) has sufficient permissions for the drive. Try creating, deleting, or moving files. If this works, then it should also work with your file transfer.
If something does not work and you are the only user of your system (i.e. security is not a concern) try changing the ownership of the drive and all files and folders there to Everyone or just give Everyone full control. This should effectively remove any permissions issues.