From the Manual (12/20/18), Pg 63 on Using Dashcam:
"Note: Tesla recommends using a flash drive with as much available storage as possible; saving the most recent ten-minutes of video requires approximately 300 MB, and the hour- long video footage loop requires approximately 1.8 GB of free space. If yourflash drive does not have sufficient storage, an "X" displays on the dashcam icon and dashcam may be unable to save video files."
Also while TeslaCam will rewrite over the 1-hour buffered space, saved files with TeslaCam (ie an accident or whatever you manually elect to save in a 10 minute file) and files created by Sentry Mode (ie while parking and triggered to record by "Alert") do not get written over so accumulate on your flashdrive until deleted. If you have a small flashdrive you may have to do this daily depending on your usage.