Exactly how do you do this?  I've been searching all menus and submenus, with both array started and stopped.  It's seeing my usb hdd, with mount option available.  Even asked me if I want to have it mounted as readable? or writable.  After doing so, I still don't see a way to add it to existing array.

 

Please help.

You cannot add a USB hard drive to the array.  You can mount it outside the array as an 'apps' drive (essentially a second cache drive without any of unRAID's automation).  Both unMenu and SNAP allow you to mount drives outside the array.  The primary purpose of this is to allow for faster data transfers, especially for a common task such as a weekly off-site backup.

Thanks Rajahal. It's called a cache drive then?

No, the cache drive is part of the array in a sense. There are multiple plugins to mount external drives in unRAID so you can copy files to it.