My name is Sneha Mangalam and I work for the Social Media and Community Team at Dell.
In reference to your query, there is a possibility that when you formatted the hard drive with MacDrive 9 the drive partition name would have been deleted and that is why computer is able to detect a drive but not able to show because there is no name. so you can try to create a drive letter. Please follow the link which will show you how to create a drive letter.
Let us know if this helps
Thanks & Regards
Dell Social Media Responder