How Do I Clear An External Hard Drive...

I have Ubuntu 14.04 LTS installed on a 8G USB Flash Drive.

How would I clear a 500G Exteal HD that coects to USB port and check it for problems and correct them, etc.

Eventually, I wish to boot from the exteal USB 500G HD instead of the 8G USB flash drive.

The 500G has some problems that need to be corrected before I can pursue being able to boot from it.

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