Lacie Mobile Drive - Scratch Disk Photoshop

Does anyone know how to link a Lacie Mobile External Hard Drive as a Scratch Disk option in Photoshop?

I keep getting an error message when opening Photoshop stating that my Scratch Disk is full. So in order to combat this, I purchased a Lacie Mobile External Hard Drive. However, when I come to change the scratch disk, the Lacie External Drive does not show up.

Any help would be appreciated.

Michael