Ok, to begin... in answer to your inquiry about the MultiSH4, it is useful in running more than 1 modded copy of SH4.
Now, with Steam, it IS possible to copy the guts from the 1st instal of SH4 from them, then pasting them into a new folder, aptly named to what ever mega mod or mods you want to run with & that is where MultiSH4 really shines...
Now, going into it further... I d/l'ed & installed SH4 from Steam, in the area set up for it, which was as I outlined earlier on... the process I went through was with Steam launcher, in going into the settings in library & telling the launcher to from then on, do all game installs to that new location...
Now...when you go to do more than 1 copy of SH4 (from the initial install of SH4... leave that copy alone from now on... as it is your 'GOLD' standard for making all other copies from, for modding...) Remember to set up the new install folder, inside of /common (I can't stress enough, the importance of doing so....).
Now...
as to the ''error message that data/menu/data/black80.tga is missing.'' thing...
There are 2 versions of SH4, that is available from Steam...
Silent Hunter 4: Wolves of the Pacific v 1.4
AND...
The release or DLC, if you will, to include that, known of as....
Silent Hunter 4: Wolves of the Pacific (Uboat Missions) v 1.5
For FotRS here, to work, the latter version or copy, is needed...
If I am NOT mistaken, that error message is sending up a red flag warning, that the copy of SH4 you have, is v 1.4 & not v 1.5....
M. M.
