Your old PC running Windows 8 has to be on the same network as your main computer. Then, just share the drive or a folder on the drive. Right-mouse click on the drive or folder, left-click on Properties, click on the Sharing tab. Click on the big "Share" button, or click on the "Advanced Sharing" button and give it a name and set the permissions (i.e., is it read-only, who has permissions to write or delete files from the share, etc.).
Then, from your main computer, bring up Windows Explorer, click on Network on the left window pane, find the old Win 8 PC, double-click on it and you should see your share. Or just type in the Win 8 PC's name and the share name like this: \\OldWin8PC\ShareName and map it as a drive on your main...