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You should check that the download is not corrupt. You don't mention your platform. On POSIX systems (linux, maxOS) you can run "type TERRA_MODIS.20190930T160001.L2.OC.nc" and "ncdump -h TERRA_MODIS.20190.930T160001.L2.OC.nc". You can get file checksums from https://oceandata.sci.gsfc.nasa.gov/api/file_search/.