I checked on the uk site and it says ...

A card reader and microSD/SD card are required to download mapping to the following devices: eTrex® Legend Cx/HCx, Vista Cx/HCx, Venture Cx; GPSMAP® 60CSx/60Cx; GPSMAP 76CSx/76Cx; Rino® 520HCx/530HCx; Astro® 220; StreetPilot® 7200/7500 (not all devices are compatible with all downloads).

So it seems it can do it...I must be doing something really obviously wrong! Is it because I haven't registered my Garmin or connected it to my PC?