An hdd file extension is related to the STSDAS software and used for its data files.
HHD file extension - STSDAS data
What is hhd file? How to open hhd files?
File type specification:
The hhd file extension is associated with the STSDAS (Space Telescope Science Data Analysis System) a software for Linux and Macintosh, which allows users to analyze astronomical data.
The hhd file stores some kind of data used by STSDAS.
This hhd file type entry was marked as obsolete and no longer supported file format.
This type of file is no longer actively used and is most likely obsolete. This is typically the case for system files in old operating systems, file types from long discontinued software, or previous versions of certain file types (like documents, projects etc.) that were replaced in higher versions of their original programs.