In reply to gammarus:
"When sending or creating files for an eTrex 10, keep in mind the following limitations:
500 GPX files
1,000 waypoints
2,000 geocaches
50 routes, 250 points per route
100 saved tracks
10,000 points per track
Ability to add maps unsupported
Each GPX file can contain a mixture of waypoints, geocaches, routes and tracks. If the GPX file containing a waypoint or geocache is deleted, the waypoint will no longer be available on device.
When utilizing routes, only 50 points can be used for on road navigation."
So, if you have some GPX files or waypoints already saved on the device, you could get over the 1000 waypoints limit with the Birketts easily, even if the list contained just a single waypoint for each Birkett. Might check that too. Especially if the limits added up together (i.e. 1000 waypoints but only 500 waypoints & 25 routes).
Anyway, with the eTrex10 limitations, it's usually better to use BaseCamp for all your waypoints and lists, clean the device and load only the few waypoints or routes you might do in one trip.