The command I'm using is:
TortoiseProc /command:diff /path:myfile.txt
It will open up the diff TortoiseSVN window, but it will only compare the working copy with the HEAD copy. What I want is to compare the working copy with the latest version in the Repo that is different. Basically, the same as running the right-click->TortoiseSVN-> "Diff" if the HEAD is different from the working copy. Otherwise running the right-click->TortoiseSVN -> "Diff with Previous Version"