If you are using GSAK to load your 60c and are using the export macro from their library it converts child waypoints in the GPX file created to regular waypoints in the GPS and over time if you dont keep them cleaned out it will finally fill up and you cant add anymore.
If this is the case you can check the box to not include those additional WP's during the GPX build portion of the macro. I havent used my 60csx in a couple of years so I am going from memory. If I remember correctly you have to manually delete those WP from the 60 as the upload wont overwrite it.
I dont store founds on my GPS after a days caching. Once I find them and post the logs I move them from my default GSAK database to a Found database. Then I refresh the caches in GSAK via GC.com and create a new GPX file for he GPS.
I keep my android phone loaded with a few choice PQ's as a back up when I happen to be out without a GPS (almost never). But GSAK is my processing medium and back those files up to dropbox and a large raid array on my PC and a thumbdrive ( I am a bit of a backup nut). My phone is never used to store anything of any value