HiCustomer Thank you for asking your question on JustAnswer.
Connect it to another computer running Vista.
let em know if it works on another computer?
Are you login to as an Administrator to Vista?
If not try after logging in as an Admininstrator and NOT a user.
Does EH have external power or is it powered by the system?
Do you have Latest Service Pack installed?
How to obtain the latest Windows Vista service pack
I would suggest you do so if you haven't. For now, see if this & the external HDD Troubleshooter from Seagate helps.
It is equally applicable to your hard drive also.