So, I'm sure we all know this one - you spend an age portal searching for the one that takes you to the NW island in the Atoll of Immolation, only to be denied on the portal off the first island.
After spending several hours on this unending hunt - and still no success - I wanted to share some observations with you peeps, which hopefully some may find useful. I stress these are my observations, without some ZOS confirmation it is speculation, but corroborated by my experience.
So firstly, lets say you enter the Blackwood portal and you find yourself on one of the other two islands, you exit immediately. At this point there is (what appears to be) a random chance that the Blackwood you return to is a different instance. You can see this by whether or not unlooted corpses remain or new mobs spawn. In such cases, re-entering the portal may take you to another island than your first attempt - because it is a different instance of the Atoll.
Lets say that this time it is the NW island. You make your way to the portal and it takes you to the wrong island. You backtrack and end up back in Blackwood, proceeding to find another portal - now here is the thing - if the instance of Blackwood is the same one as before you'll end up on the NW island BUT it also means that the portal off the island will take you to the wrong one. In other words, the instance of Blackwood you enter is linked to the instance of the Atoll. There are ways to determine which Blackwood you are in. Don't loot the Dremora corpses. Note what types they are, and you have a better chance of knowing which instance you are in.
Simply running around like a headless chicken is - put simply - a waste of time. As you are simply entering the same Atoll's over and over, none of which have the correct portal off the first island. Of course eventually you are going to hit the right Atoll, but my own experience strongly suggests that if you haven't found the correct Atoll after 10 portals you won't find it anytime soon - at least until the instances reset, but I have no idea how long such a reset takes.
The reason I say 10 is because I have tracked three instances of Blackwood/Atoll that I'm put in during a game session - including moving in/out of portals, and there are three starting locations in the Atoll.
My suggestion to anybody attempting this monumental effort, to get a simple open world quest, is basically do 10 portals a day. I would suspect that 24 hours is enough time for the instances to reset/refresh. You could spend 8 hours in a row, but from what I've seen (as described above) you are wasting your time.
Thanks for reading,
Gab