Partition and erase external hard disk failed-- "operation failed"

I'll just describe the problem that I had with my hard disk and how it occurred.

First I'll show you the details of the hard disk:

- I got this hard disk( Sony 1TB 2.5" External Hard Drive) nearly a year ago. And only used it on two of my macpros to back up the datas and files.

- Recently I tried to let someone copy a video for me from his PC, but he said that his PC couldn't read my hard disk. Then I came to realise that the disk wasn't setup for both system(mac and windows).

- The person didn't know what was the problem also so he let some else to fix my hard disk on windows and obviously it didn't work.

- I asked it back today and try to erase it to MS-DOS (FAT) on my (early 2015) mac pro(OS X El Capitan 10.11.1), but it says erase process has failed.

- Then I searched online and followed one of the video which says go to the partition and format it to MS-DOS FAT, then go to erase and do the same thing as described above. However, it didn't work as well and shows "operation failed"

I'm wondering Is this problem caused by the operation on the PC? I don't know what exactly the guy has done because I wasn't there. Or maybe the hard disk is damaged that I have to restore it. Anyone here could help me?