1. is the drive spinning without clicking noise? If no, check cable on both external drive and computer. Is the USB port good with another drive?
2. if yes, do you see the disk under device manager/windows and system profiler/mac? If no, reboot or unplug-replug.
3. if yes, do you see the partition under disk Management/winows and Disk Utility/Mac? If no, check if security enabled?
- if yes, do you see the drive letter assigned/desk management/windows? If no, right click and assign drive letter.
5. if yes, you may have to format the drive again. **** Do this after all other methods failed as it will wipe out your data.***
