yep, ivan and scottie are correct. do you know what kind of hard drive is in the unit ?
seagate and western digital both have free boot disks on their support websites you can download, and run diagnostics on your hard drive. if you dont know , just download the one from seagate, it will still help with non-seagate drives, and at least tell you what kind of drive you do have.
here it is:http://www.seagate.com/www/en-us/support/downloads/seatools/