Just sharing what happened and how I solved it.
tldr; Activities can be saved with a '/' in the name. When you access saved activities the GPSMAP 66i will crash and reboot. Don't use '/' in named activities (or elsewhere I suspect).
Saved an activity as "HP 06/08". Instead of a saved activity by that name, in the /Garmin/Activities folder on the device, a subdirectory is created called "HP 06" containing a single file, "08.fit". If you go to the menu and then select "Recorded Activities", the device hangs and then reboots itself. Solution: Mounted the device on my computer. Examined the "/Garmin/Activities" folder, saw the subdirectory and immediately understood the issue. Moved the fit file from the subdirectory to the "Activities" directory. Renamed it HP-2020-08-06, removed the sub directory, I can now see recorded activities again with no crash. It would seem that the device should not allow names with slashes as this problem is unsolvable using just the device it would seem.
