I realize now that if the external USB drive connected to the NAS is formatted NTFS then it can only be read from, not written to. The manual says that the drive needs to be formatted FAT or FAT32 so that it can be used for writing to. Unfortunately, FAT32 does not allow a file more than 4GB. Current HD multimedia files can easily exceed 4GB.
I see that the internal storage is formatted XFS. My question is this. If I were to somehow format the external USB drive as XFS, can it then be used to write to?