If an USB hard drive is attached, it will be recognized without the need to reboot the system. The "Found drives" during system startup are listed in the upper section of the ExtraHD configuration page. These include the partitions used by the system itself.
The second section lists all hard drives recognized during boot or which have been connected via USB.
In the example shown, two additional hard drives have been found.
If they are already formatted with the filesystem you need, they can be directly mounted into the running system.
To include the hard drive into the system edit the name field of a partition belonging to the disk. This field contains "/mnt/harddisk" ( which is the mountpoint of the system disk ). Choose a unique name and hit "Add".
If the additional hard drive(s) has/have been successfully mounted, they will show up on top of the ExtraHD page.
Mounted and existent ExtraHD are in green, partitions defined but not connected are shown in red. Partitions are identified by UUIDs. Therefore if you plug in a USB disk with partitions already defined, these are mounted automatically.