Connect to your 660 to your PC via USB, the unit itself, not the SD card.
Look in the folder /Garmin/GPX/Archive/ and you will find your full history of track-logs from the day you first switched the device on. Once the current track-log history is full it get's archived there. Files are in GPX format and once opening in Mapsource you can see them and related data like speed. time of day, elevation etc for each track-point along the way.
Obviously there is a space issue after many thousands of miles but on my GPS I've done a measly 7,000 miles since last winter and there's 10MB of track-log data backed up in that folder. It will be a while before there's a problem with the memory filling up as there's 550MB still free with the very latest mapping loaded.