You are not alone. I have 2 640s, and am on the windows insider program and have the one of my 640s on the Release track and the other on the fast track. The one on the release track is my original T-mobile phone and I use it as my regular phone. It is doing exactly what you describe. I can take 2-5 pictures and then it will say saving. It will say saving for days if I left it. Eventually I have to shut the phone off and turn it back on again to get saving to go away. I cannot take any other pictures when it is on saving. When I open it back up the pictures are "there" the folder but no applications (FB/Instagram) can "see" them. The pictures are saved with a .thumb extention. This happens randomly as well and sometimes pictures I take before I get the Saving... issue are also saved with the .thumb extention.
It is infuriating. I was away this week for my anniversary, and had to reboot the phone numerous times. I ended up having to use my work iPhone 5s to take pics because the 640 was so unreliable. Now, I know, I am on a release preview, so I can't complain too much, but I wanted windows 10 and T-mobile just walked away from it. I did try the same thing on my ATT version, unlocked 640 running on the fast track and this doesn't seem to be happening. It does say saving for like 10-15 seconds or more in HDR mode, but eventually it goes away and the pictures save without the .thumb extension. I ended up having to rename a bunch of photos in the file explorer and remove the .thumb extension to be able to share them with family.
I was so ticked off, on the way home from our trip, I ended up buying a clearance Moto G at Best Buy just to start playing with it. I am not a fan of Android, but Microsoft is making it more and more difficult to be loyal to the brand, as much as I love the Windows interface on the phone. They aren't even going to sell Microsoft band anymore. So again, Windows folks are left high and dry. .
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