It is not connecting right. It could be your computer - or it could be the drive. First thing you should always do is try a new cable - if that doesn't fix it, you try another computer. There are 3 possible failure points:
1) The Cable (replacing it eliminates that as the issue)
2) The computer (trying a 2nd computer eliminates that as the issue)
3) The drive - when it doesn't work on multiple cables and multiple computers, there is an issue with the drive.
One you identify what the cause of the problem is - you can attempt to repair it.