You said that the hard drive shows up in Disk Management as online, but is the partition on the hard drive assigned a drive letter? That's caught me out in the past.
Try locating the hard drive in Disk Management again, right click the partition, and press "Change Drive Letter and Paths...". In the window that appears, check that there is a letter in the box. If not, press "Add...", and choose one.
Let me know how it goes.