If you download a .rdr file, standard Mac tools like Disk Utility may not open it directly. You will likely need specialized software: Download macOS High Sierra 10.13.5 Update - Apple Support
: You can find various macOS High Sierra images, including ISO formats , which are useful for virtual machines. How to Use an .rdr Image File
The .rdr extension is not a native Apple format. In the context of macOS downloads, it usually refers to one of two things:
: Formats developed by R-Tools Technology (R-Drive Image) for system recovery and hard disk imaging. Official vs. Unofficial Downloads
: You can often find High Sierra through a direct App Store link that may not appear in standard searches.
Apple generally provides the of High Sierra (10.13.6) through its official support pages.
: Used in communities like ihackinOS to provide a full macOS installation that is bootable immediately once restored to a drive.