iMac OS USB install problems, Yosemite to High Sierra
I have a Late 2009 iMac that i purchased second hand in 2015. I am trying to upgrade the OS from Yosemite (10.10.x) to High Sierra. The install fails, and I have spent days trying to find a workable solution. Yosemite was on the machine when i bought it, and I doubt if it's listed in my appstore purchases (it was bought second hand).
Initially i backed everything up onto an external HD (no Time machine backup), downloaded the High Sierra.app installer (the small partial one), and ran it hoping that the upgrade would just work. During this process the full installer downloaded, the system restarted, and it stopped with the error "MacOS couldn't be installed on computer. Installer resources couldn't be found." At this point I created the USB installer via terminal (createinstallmedia) on a 16GB stick. I created the USB from the .app applicationpath and following the instructions from part (b) discussed here:
The only notable mention is the step of copying the contents of the full Install directory into the Contents/SharedSupport/ dir. Anyway the USB successfully created. I booted into recovery mode, and formatted the destination disk as Extended (Journaled) with GUID scheme per the usual instructions. Now when i boot to USB (holding Option, which goes automatically to Recovery screen), i select "Install MacOS" option, click Continue, and then nothing happens. The machine isn't frozen, it just doesn't advance into the installation.
I open terminal and poke around. I try
hdiutil verify "/Volumes/Image Volume/Install...High Sierra.app/Contents/SharedSupport/InstallESDDmg.pkg
and it comes back valid after a few minutes. One thing I have noticed is that there is another root copy of the Install high sierra.app in root (/.), which does not have the SharedSupport dir.
I would be truly appreciative of any advice. My only other device at the moment is an ancient chromebook. I've tried every available piece of advice I've found in the past few days, and I'm still stuck.