External mac hard disk is normally used to make the absolute backup of the mac system with Time Machine tool. In case of the data loss and system failure, you may attach an external time machine hard disk and then restore files from it.

But in some cases there is a possibility that both external Time machine hard drive and internal Mac hard drive can fail at a same time. Then you do not have any other way to restore or access the data. Such cases may include:

When you will form an internal mac hard disk and then reinstall the operating system, an external hard disk is attached to the system. It can damage the file system and structure of the external drive.

The internal hard disk is infected by the virus and external drive is also attached to the system. Then external drive can get also affected by virus. Then you can face the hard disk logical failure.

In all these cases you need to perform the file recovery mac which is possible with powerful and professional data recovery mac software.