In case anyone else has this problem, I will share my solution.
I have a Oregon 550 and BaseCamp keept telling me an update was available every time I started the application. When I allowed the Web updater app to run it got to 99% then complained there was not enough free memory to finish the update process and left me at V4.80.
After removing all my waypoint and tracks and the Micro SD card it still showed only 66K available.
The update needed about 10M so this was never gonna work.
Up to this point I had been using a MAC running OSX 10.6 for these updates and for base camp.
So I thought why not try a PC and see if I get the same problem, and it was the same on the PC still complaining about not enough memory for the update to V5.10.
However when I used windows explorer to check the memory contents it showed the .trashes file that the MAC had created to store deleted items.
The .trashes folder was over 800M, so I deleted it and then I was able to completed the firmware update and restore all my original waypoints and tracks.
Not sure if I will go back to using the MAc at this point but it least the Oregon is useable again, and with the latest firmware.
Hope this helps some else having similar problems.
Peter B.