I have very stupid problems with my Selfmade IDE cable. A friend will make me new cables because at the old one connector is damaged. I have some problems with it:
i think he has try this http://www.xbox-scene.com/articles/multi-drive.php tutorial to made the cable, but which measurement method is in that tutorial?
i have problems with the conversion. How long must the Cable in centimeter? from the start to the end. And between the Connectors?
The Problem is that only the third connector makes trouble with the data transmission (i have tested it in my Computer with a Power Switch for the HDD'S).

Can anyone give me the correct lenght for the Cable?

I have heard that there was a length limit to IDE cables of like 3 feet or something, but other than that, there is nothing special about the lengths, or connector spacing. Standard cable length is about 18 inch (I don't know how many centimeters that is), so I would stick as close to that length as possible. As far as having a 3 device IDE, it is a bad idea. You are just asking for problems there.

No there are no problems with this, it is a relais between the HDDs. i can switch between them so that one becomes power the other not.

Google will compute the inch to centimeters for you:
1. 5 inch = 12.7 centimeters
2. 10.5 inch = 26.67 centimeters
3. "9.5inch in cm" , ie 24.13 centimeters.

Some time back, I replaced the stock 40 wires cable with a 80 wires one that came with my PC motherboard.
You'll notice that the XBOX one is slightly longer! So I had to fold it in a V shape instead of a U shape like the original.
After a while, my cable got damaged around the folds, and my HDD started playing up (i.e. it wouldn't boot every other time), so I reverted to the stock cable: now no more HDD problems, and TBH, the loading times difference is minimal. So all in all, if I had to consider doing this mod again, considering the hassle and the marginal performance gain, I wouldn't bother.

You are just asking for problems there.