Apple making things easier
So you tried to install High Sierra but nothing happens? The installer has the old "dirty unmount" bug so hfs_fsck is busy fixing your boot disk while the installer is looking for it. There's an easy fix.
Download the High Sierra installer over and over until you get the full 4+ GB version. Insert a USB stick and reformat it. Find the secret terminal command to convert the installer to a bootable volume on the USB stick. Back up your hard disk using Time Machine. Remove the factory hard disk and install an SSD or very fast hard disk. Boot your base OS DVD and install onto the new boot disk. Boot into High Sierra from the USB stick and update the boot disk. Reboot and restore from the Time Machine backup. Run Software Updates repeatedly for the next several hours until it's up to date.
So easy! Only needs a few days of work, an expensive new hard drive, and about 15 GB of downloads.