What is the difference between normal sampling mode and fast sampling mode?

Normal sampling mode does not need the external power supply (supplied by USB port). You can also download data from logger while the logger is still logging.

If the logger is logging in fast sampling mode (sampling interval is below 5 seconds for Site-Log, iLog Strain Gauge and 1 second for iLog single channel loggers) it needs an external power supply (supplied by a USB port) for the logging period, and if the logger detects a communication request it stops logging. So please do not communicate with the data logger while the logger is recording in fast mode.

The proper way to start a fast logging session is to add a start delay when you configure the logger and apply the external power supply to the logger before it starts logging.