Control Overwrite Behavior in Export
Export into a non-existing file.
Exporting new content into the same file overwrites the old file.
Use the OverwriteTarget to avoid overwriting existing content.
In addition, using OverwriteTarget"KeepBoth", Export automatically chooses a suitable name for the new file.
Some file formats, such as HDF5 and FITS, also support OverwriteTarget"Append", which allows adding data to an existing file.
Export a single image to a FITS file, then append another image.