After failed cloning - target disk not detected by Windows

Using Windows 7 and ATI 2014. I wanted to copy all the files from a 1T Seagate Portable External Disk to a 2T Backup Slim Drive. I used the cloning feature. It went on for 4-5 hours and then failed with error message related to Read/Write Errors on both disks (Error 41527 I think). I chose «ignore all» and things kept popping up for different sectors on the disk. I then canceled. The source disk was not affected, but the backup disk is no longer recognized by Windows so I can't even reformat it. When I connect it in the USB port it is making the normal connection sound. In Windows Disk Manager utility it appears as «base» disk with: 100Mb partition for the system, the second partition is 1862.79 Go «not allocated» it is shown as Black.
Does anyone have an idea? This was my first attempt at cloning.