Thanks Alastair and Alex for those comments. You have prompted another 2 questions - (a) I had assumed that the FS program was loaded entirely at start up, so that the only disk accessing going on during flight would be scenery loading. Can anyone confirm this? (b) You both prompted me to have a good look at the specs, and clearly it is highly desirable to use USB 2.0 rather than USB 1.1. My elderly (3 years old) Dell PC has a number of USB ports, how can I tell what spec these are? Note I was thinking of getting one of these disk drives to store the Horizon FSX scenery. Frank T.