I've also noticed, if I grab the esd file when it still says "downloading", (even if it's at 100%), and then grab it again when it says "Preparing to install", I get 2 different esd files - and they are exactly 1 byte different. One will convert to an ISO and one will not.
Mine is there as well as shown in my earlier post. All 2,668,973 KBs worth.
I see the OP came back on. We'll see if he found it.
When you open Sources, there's an alphabetical list. You have to scroll down to the Install.esd (about 3.5 full mouse wheel turns).
This thread is a couple of days old. You probably have the issue fixed.
But just in case you are still looking for Install.esd....................
I had the same issue you had. I installed build 10576 via Windows Update.
It has been my habit to use the esd file and convert it to ISO and then burn it to a DVD. I do that to all Insider Previews as well as the RTM.
But this time, when I opened C drive, there is no $Windows.~BT or WS. ( see screenshot #1).
I went to File Explorer Options > View > put a dot at Show hidden files, folders and drives. Opened C drive and there it was $Windows.~BT. I opened it > opened Sources and got my Install.esd.( see screenshot #2.)