Stellar Data Recovery will display all drives that your Windows OS can recognize. The source of problem could be a damaged power or data cable or the external drive itself. You can follow a few troubleshooting steps to fix not detecting hard drive issue:
Verify the Power and Data Cables: You should verify that the power cable is properly connected to the external drive . If you have used an extension cord to connect the power adapter, make sure the connection is working fine. Also, check your data cable connections on both sides. If the problem persists, try connecting with different cables.
Verify the Drive in Disk Management: Sometimes, a drive is not seen in 'My Computer', but it shows up in Disk Management along with the problem listed on it. If the partition format is listed as 'RAW' or the drive shows as 'Unallocated', this implies that the drive partition is corrupt. To work around this issue, you can recreate the corrupt partition in Disk Management.
It may happen that the drive letter assigned to this drive is used by some other device. To correct this, you need to manually assign the drive letter in Disk Management.