First of all **always use a virtual machine**  instead of your installed OS **and mount it in read only mode** when doing something with (probably) infected externals!

This prevents your PC/Account from being infected by whatever is on this HDD.
Most likley a usual anti-virus should then be able to detect most of the malware and delete it.

If you doubt it anyway you can completely wipe the HDD using the VM.

In addition, there are already posts on this topic:

 1. https://security.stackexchange.com/questions/35420/safe-to-connect-to-external-drive?noredirect=1&lq=1
    
 
 2.  https://security.stackexchange.com/questions/64404/mac-osx-what-is-the-safest-way-to-access-an-unknown-usb-storage