If you have downloaded Install macOS High Sierra.app on a non-macOS 10.8 machine then copy said installer to a 10.8 Mac, the installer will be unusable. Why? HFS Compression. It seems Install macOS High Sierra.app uses HFS compression on numerous files and it seems 10.8 has issues with that and will not “see” those files.
To quickly demonstrate which files are compressed, we will use stat and examine the st_flags attribute, 0=no compression, 32=compression (thanks ghost of Mac OS X Hints!)
IFS=$'\n\t'; for file in $(find /Applications/Install\ macOS\ High\ Sierra.app -type f); do echo "$(stat -f %f "$file"): $file)"; done
You’ll see many files with 32 as the flag, these will not be readable in 10.8
The trick is to copy the file and do away with the HFS compression, we can use ditto for this:
ditto --nopreserveHFSCompression /Applications/Install\ macOS\ High\ Sierra.app [destination folder]
This will produce an installer compatible with 10.8… sure you could have downloaded it via the Mac App Store by now on a 10.8 machine, but now you know.