Assalamalekum,
Regarding your external hard disk issue, the fact that it is not being accessible in more than one computer probably means that the boot sector of the primary partition of the hard disk has been corrupted, hence it is not booting up. In such cases, the chances of repairing the disk to working order are slim. Also, every time it tries to boot up, the disk will get progressively more corrupted. As I see it, your only option is to recover your important files from the disk before it dies. There are several commercial software to do this but it is expensive. It is difficult to suggest a solution without physically checking the disk. If you really have important files that you want to recover, I can suggest a plan of action and the expenses involved.
Regards,
Ahsan