Sorry I was just updating my previous post with a few additional details while you replied.
About the (screen)freez though: it works like an amplified version of an effect that I always had when turning on my (old) external hdd: the moment windows detects and accesses it I used to get a very brief hickup, as in the screen/pc freezes, for about 1 sec. maybe less. Now if i switch the hdd on a 2nd time i get the previously described intensified version of this, as if the pc is trying to unsuccessfully access the drive a few times before giving up.
That however does not happen when turning it on the first time.
Combined with the other symptoms added to my previous post I am still somewhat confused.
Either way, I will most likely leave it the way it is now. But in theory, if wanting to go back to the old setup/situation, would I just have to switch off AHCI in the BIOS or also manually remove/turn off the newly activated AHCI driver in windows?