FINAL UPDATEDear all. This is an update. I finally got trough a full reinstallation process of Sampletank 4 MAX and all its derivatives (Syntronik, Sampletron, Mirosplav Philharmonik). IK Multimedia team had made a belated but kind proposal to me (I do not want to enter into the details) but I refused because I really wanted to understand how it works in depth. So I managed to check all the zip files I had kept in store and I tried a full step by step customed installation.
The reason is that I wanted to avoid the bad experience I have had with the IK Product Manager. The manager is a black box where you hardly understand what it is doing; things are sometimes unclear where the files are put, where they are decompressed, where they are installed, and where the process has stopped for all sorts of reasons. It may be of help for a few files but when things go the wrong way with connection errors and installation errors, the more than one hundred files and hundreds of Gigas of data let you stay in the fog, not to say that your disarray lasts for days and days of downloading.
Nethertheless, my customed installation encountered also many problems. I had to face some "Could not install the library" or "Could not install some files samples" issues. I know now for sure that this problem comes from a specific difficulty with the lenghth of the paths when installing or dezipping. So everyone making a customed installation SHOULD put the zip files at the root of a hard drive and make the unzipping and proceed to the installation from here, thus reducing the lenghth of the paths during those processes, and those problems will be avoided. It's not clearly stated anywhere but it will really avoid you much trouble!
I also noticed that previous choices concerning the installation path (Sampletank 3 or Sampletank 4 locations) has an influence on following installation paths. So it may happen that the installer proposes you to install the files at the wrong place; and guess what?...when it has finished, it claims that the installation succeeded, although in fact, nothing was installed! Moreover, the label of the zip files does not always properly states which type is it, ST3 or ST4. For example, how do you know where to install Pedal_Steel_Guitar_Sound_Content.zip ? So check the answer by inspecting the inside of the zip files and you will (sometimes) have the answer. ST4 files are *.st4i files while ST3 files are *.st3i files. For the .pak, files, use your thumb to guess: the name of the Folder "Sampletank 3" or "Sampletank 4" also helps.
I had also some minor difficulties. My previous copies of the sound folders showed that the final new result is a bit different from the previous result.
For example, no "ST4 imported" folder was created while it was present before. Creating this folder manually makes something curious happen: hence the folder is empty, when clicking on the library "ST4 imported" inside the ST4 software makes the "Piano Felt Pedal" instrument appear, which instrument is absent from the "Fractured piano" Library where it shoud be located! Probably are the data of the libray info wrong. And if you look at the dedicated picture on the relevant IK multimedia page here
https://www.ikmultimedia.com/products/stfracturedpiano/ you see that the very same instrument is missing from the list in the software screen picture: it should be located inbetween "Piano Felt Bass Pluck 4th" and "Piano Howlings B" and it isn't. IK multimedia teams, check your files on your own computers, ah, ah !
My new installation also showed a lock upon a "Syntronik-CS-Free" library; and digging among all zip files couldn't locate any relevant file to this "Syntronik Free" instruments. But, since I kept a copy of my previous installation, I could locate the existence of previous "52 Syntronik Free" folders in the Sampletank 3 samples folder and in the Sampletank 3 instruments folders, as well as a Syntronik Free.pak file inside the ST3 Libray info. And putting those files manually at the appropriate place in my new folders could resurrect a few previous instruments that came, I think, from a many years free old Syntronik version, and which are not available anymore (at least with a custom installation made through files downloaded from the "My Products" page). These sounds are named "FM Tubular Bell Light" and "Classic EP 3 Velos" [99 synthetizer] or "Glisten Light" [Blau synthetizer] to name as a few.
I would also like to point that explanations about processses to follow when making a custom installation differ widely depending of the date of the PDF files or the web page that you follow. Some explanations are dated of the old "Custom shop" system; and schemes of the folders are also sometimes old dated and do not reflect the up-to-date structure of the software.
Despite all those problems, I urge IK multimedia to preserve the ability to set a customed installation: it helped me to understand how ST4 works, how to change things manually and properly, how to make my installation evolve smoothly from one location to another, how to stay versatile when allocating space for ST4 derivatives, that I may put on different hard drives, depending of my everyday use of them; and it solved my installation problems much more better than being stucked with automated installation problems.