imac 2009
Unable to load MacOS High Sierra. The computer stalls half way through.
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No error messages?
What 2009 iMac? Only the Late 2009 iMac can run High Sierra. The Early and Mid 2009 iMacs are contrained to no higher OS than 10.11 El Capitan.
If yours is a Late 2009 and doing this, it could be a hard drive issue. Either it is too full for the installer to work or it is failing after 12 years of use.
If you downloaded High Sierra a while ago, it may be a unsigned outdated version. You can get a fresh version of installer from the link in this Apple article:
How to get old versions of macOS - Apple Support
Be sure to use Safari for the download; FireFox has not been happy with macOS system installers lately. Also make sure your have removed the older installer version from your Applications folder.