I was looking for another previous post in past. But I think it will take a long time to find and show you. So, I will tell you some about it.

Vowel harmony is based on the pronounciation. Not the spelling/writing exactly. some vowels have a bit different sounds from which they are supposed to have. For example, in "at" (horse), you definitely hear the Turkish letter "a". but in the last syllable of "sadakat" or "þefkat". these are not Turkish hard/bass "a". They have a sound between a and e, which makes them lighter. Meþgul´s u has also a sound between u and ü, and that makes it lighter than normal. For that reason, you cannot go ahead with -u- style vowel harmony but you have to go with -ü- style. which is like "meþgül müsün?", but sure that is not an "ü", so this is needs a trable/soft vowel: "meþgul müsün?"

other examples; anne þefkat+i

Bize sadakatini gösterdi.

etc.

Thank you for taking the time to explain it, I think I understand it much better now.

I read somewhere that meþgul is a borrowed word from arabic. Some of the arabic words don´t exactly follow the "Turkish" rules (methinks).

Thank you.So you have to know how it´s pronounced as well as just knowing how it is spelt-how very confusing for the non-Turkish speaker!   

Every language has exceptions. There are a few exceptions for this and no one needs to worry about these