First post.
I spent the better part of four hours researching and trying to solve this problem. In my case I had four apps that had updates. I would download the update and it would go to install them, and then it would give me the black triangle of angst and say it was unable to install to the sd card.

I found the same article regarding the fix in many places. Having to look for a file in one of two folders, the path of one being unaccessible unless rooted. Of course the easy to get to one was empty.

I did the move app to phone, update, then move back to sd card shuffle. Pain the you know what. I just deleted two apps to save me the hassle.

I then went to lifehacker and did the root job on my droid. Crossed my fingers and it worked. I just now got into the sacred folder using ES File Explorer (after turning on the root feature in settings) and I found the bugger!! Last entry all the way at the bottom... of course. With it gone I am fairly confident of no more problems, but I have heard that the problem can come back.

This has a lot to do with interruptions during the app update sequence from what I have read. It makes sense as I had my phone reboot on me in the middle of a lot of updates a few days ago. Think I have done the last of my mass updates. From now on, even if it is a minor pain, it's one at a time.

I hope you get this sorted. With any luck it will be in the folder you can access unrooted. Good luck!

dune.