After playing around a bit more, these are my conclusions:
(1) The 1 second and maximum sample rate settings are not usable. They both log about 3 readings/sec which are recorded in multiple files with time gaps between them.
(2) the 10s and 1 m rates work OK, but logging starts at the 3/s rate when logging is turned on, than reduces to approximately the nominal rate when back button to the Scan screen is pressed. On return from the Scan screen to the Meter screen, sampling goes to the 3/s rate. All the reading are recorded in the file, but those recorded at the nominal rate are separated from the others by a repeat of the headers. Even when the back button is pushed ASAP after the logging is turned on, 10 or more samples will be taken at the 3/s rate and will appear at the start of the file.
(3) The actual rate on the 10s setting ~11.059s and on the ~61.330s on the 1m setting with standard deviation of about 30ms. If you need to be accurate, using the time stamp values is important.
(4) AS others have mentioned, the flashing LED is useful as indicator that sampling is OK. On the 10s and 1m settings, it flashes each time a sample is taken. On the (not really functional) 1s and Max settings, it is on almost continuously but cycles through a pattern of brightness variation every 2-3s.
Suggestions: A software update that doesn’t initiate logging until the back is pushed would be a major improvement. Presumably there is a sampling rate faster than 10s that could run reliably without producing multiple files. It would useful to determine this and add modify available sample rates accordingly.