EDIT-- This is for Windows. I don't have any ideas for an equivalent Mac program. Since the Garmin file has no line feeds, it's messy to edit with a text editor. But if you know the starting and ending times you want to keep (adjusted for Universal time's hour offset), you could just locate and delete a group of Trackpoints.
A .csv (comma separated values) likely doesn't keep the same structure as a gpx or tcx file, so it's difficult to convert back.
My Garmin 705 occasionally mangles a small part of my .tcx recording, but that's enough to keep the whole ride from loading.
I've used the free Firstobject XML editor
to delete the bad entry. (Save a copy of the original file first!)
Garmin tcx files don't have any line feeds, so the file looks like one extremely long line of data. In the XML editor click Tools-->Indent to display it in a readable format. Then in the left hand column, expand the Activities-->Lap--->Track sub headings to see each individual trackpoint. Doubleclick a trackpoint to highlight it. The time is in Universal/Greenwich time, and the distance from the start is in meters.
You can easily delete data by selecting, in the right side detail view, a starting "<Trackpoint>" and deleting through an ending </Trackpoint>. Use the Delete key on the keyboard.
I use 1-second recording, so there's a Trackpoint every second.
Last edited by rm -rf; 12-01-13 at 09:39 AM.