Notes about the HDK file extension:
To open this file type, a required software must be installed. Without it you only see a Windows message "The following file cannot be opened
" (Windows 7) or "How do you want to open this file?
" (Windows 10) if you double-click the file. There click on "More apps"
and choose a suitable software to view such files on your PC or from email attachments.
The HDK file type is primarily associated with 'MAXI Disk' by Herne Data Systems Ltd.. It's believed this program will open this disk image format but not confirmed.