How to recover data of Hard Disk with many bad sectors?

Have you ensure the USB /SATA cable are good, meaning not loose when inserted.

If you are using external box casing to slot in HDD and then connect by USB /SATA try another external box casing.

I encountered this a couple of days ago, and after getting a new USB casing, there is no problem.

Do note that HDD must be either horizontal or vertical, not at a angle.

Yes, I've checked both the casing as well as the connecting cable. I went there from where I bought the casing. They inserted a new internal hard drive into the casing with same cable & that hard drive was showing in 'My Computer' as a new partition. So, the casing & cable are fine.