Accidentally used "cp /file /dev/sdb1"

Solution 1:

With the command you used, you have overwritten the beginning of your external hard disk partition with the copy of the Windows ISO, because you copied directly to device instead of the directory the device was mounted on. Thus you have destroyed the important filesystem structures on your HDD. A specialized data recovery software might help you to recover something from the part of the disk that hasn't been overwritten, but it is not guaranteed in any way. Sadly, your data may be lost.

In the future, never use device names in regular file management commands like cp. If you use a desktop Ubuntu installation, there's rarely need to use device names directly anywhere. When you plugged in your HDD (while it was still OK), it probably got automounted under some directory, something like /media/userid/something. This directory usually should be also automatically opened in a File Manager window. You should have copied the ISO file there and not to /dev/sdb1.