Lost space after diskcopy

I had a 160Gb HD that was failing with bad clusters. Every time I used it, it came up with more bad clusters/sectors. So I bought another 160Gb HD and did a disk image. From this new HD I managed to get my important files back. Since I have all the files I need, I went and re-formatted the new HD so I could use it for other purposes.

However, when I fomart it shows no data on the HD but is showing 21Gb of space being used with 127Gb free.

My guess is that disk image copied everything across to the new HD and even marked the bad clusters as 'bad'.

Obviously these 21Gb of 'bad' clusters on the new HD are not really bad. How do I get that space back?

There are different hardware diagnostic tools available online. Download one of the tool and run the tests on the hard drive. The result of the tests will show what exactly is the problem with the hard drive. If the hard drive is inside the warranty period then take it to the nearest service center of the manufacturer and get it replaced.