Mirroring my verbose answer from the help-platform that some more people may have benefit from it.
- No, you cannot set up newer Garmin devices to write just one gpx file.
- If you want just one file to upload: zip it.
- For maximum verbosity of the log files, either set the log interval to secondly or the distance to 0.00
Old Garmin devices with micro sd slots were designed to write one gpx logfile per day. This file contained the segments which resulted from switching the device off and on or (iirc) when the device wrote it's buffer of 9.999 waypoints to disk.
The newer devices as of ~2010(afaik) and later save the gpx log to their internal memory. The device can set to "automatically save" it "daily", "weekly" or "when full" but I see no sense in this setting since it only names the files after day or week and creates a lot of them. Additionally the created files also contain several way segments.
I manage the gpx files this way (the device being connected to the computer with mini USB cable):
Samat K Jain wrote a gpx splitter which extracts the single way segments from the gpx files created by the Garmin device.
I wrote another small script which copies the gpx files from the Garmin device (both from GARMIN/GPX/Archive and GARMIN/GPX/Current), splits them using the splitter script and saves them to ./. The waypoint files get copied, too.
Reasoning to split the files in even smaller chunks: A gpx file named 2013-12-20 12.54.09.gpx is named after the date it got saved. It can contain way segments from other days or even weeks depending on how often the device is used. The splitter script gives each way segment an name like 2013-10-20T14:31:48Z.gpx
Another reason to use the splitter:
The latest collected data is stored in GARMIN/GPX/Current/Current.gpx. When I copy this file to my computer and do not reset the GPX log buffer of the Garmin next time I'd copy the Current.gpx I would have two files with identical, non-descriptive names and partly identical content. Using the splitter I avoid this.
One more advantage of having the way segments separately: having files of 1-2kbyte size you can easily delete them because files of this size usually contain no useful data. (YMMV)
GPX data I want to upload to OSM I mostly can define by the date, so I just run
tar -caf hike_november.tar.gz 2013-10-2[0-5]*gpx
to create one file I can upload. Yes, OSM accepts zip and gz files.
Another way to have just one gpx file is to append the gpx files you want using gpsbabel. I wrote a small script for this, too.