Q: AppData\Local\Apple Inc\CloudKit\iCloud Photos\MMCS folder  is using over 250GB on my hard drive, can I delete it, what is it there for ? offline viewing? If so I don't need it

I had the same problem and it chewed up all my disk space. I have a Windows 10 64 bit machine - 250 gig drive. I deleted all of these files except for those with today's date and freed up 150 gig. Much better to delete these temp files than to risk an operating system crash from running out of disk space.

I have a customer's computer (Windows 7) that ran out of space on a 1TB drive. The "iCloud Photos\MMCS" folder is 777GB!! There's only 72.4MB free on the drive now. I pulled it and ran WinDirStat on the drive and found a happy surprise in the \Apple Inc folder.

There are 1,310 files in the folder dated from 8/24/16 to 9/12/16. 256 of those files are larger than 1GB each. I fear if I delete all those files it'll be back to full again in 3 weeks.

Why does this happen? Is there a setting that can be turned on to automatically remove those files?

From what i have learnt over the last couple of days.

CloudKit\iCloud Photos\MMCS is filling up nicely.

I kept freeing up space on my hard drive only for it to vanish  a couple of hours later and using spacesniffer it clearly shows that CloudKit\iCloud Photos\MMCS it hogging all the space. I even used the windows diskcleanup software which in all my years of computing i have never done before, I was amazed how much room i got back. Left it overnight and CloudKit\iCloud Photos\MMCS had filled up another 25Gb !!

I recently took some videos on my iphone of lightning and the files sizes are quite big so i decided to copy and rename a tmp file in the CloudKit\iCloud Photos\MMCS folder to .mp4 and as i was expecting it was a copy of one of the videos.

i used icloud for photos and it copies photos and videos from my iphone onto my computer. I have all these files in my Icloud photos folder. so thats working ok.

I have read somewhere that icloud still needs a copy saved on your computer as another backup for the icloud. Dont know if thats correct but does sound plausible however it would be nice to know if this is true. This would explain why the CloudKit\iCloud Photos\MMCS is so large. Im not sure if this is only for icloud drive (don't use this feature at all)

I did have problem before where icloud photos was not downloading my photos and i had to manually download them to start the ball rolling. From then on the service seems to have worked. However when i did manually download i had 2 copies of everything in 2 seperate folders and had to delete some as it was taking up space and also it only updated one folder.

So it may be a configuration issue and it doesn't know what to keep and what to delete amongst the TMP files within CloudKit\iCloud Photos\MMCS

Now looking at this case, this is just 1 phone that its backing up. So is only containing files from a single phone. Now if you had a couple of iphones and a ipod and maybe a ipad. the amount of files could be 3-4 times that of a single iphone so the CloudKit\iCloud Photos\MMCS folder could be huge and deleting it would be silly unless you were sure you definitely had a copy of every file you wanted to keep.

OK, my next move.

What i am going to do is take all my photos and videos off my iphone (needs a de-clutter anyway) and make sure there are none left on my iphone and all saved on my pc.

Then i'm going to perform another Disk Clean and use CCleaner and just check everything has gone and see what happens overnight. Hopefully there will be nothing to download from my phone and CloudKit\iCloud Photos\MMCS will not fill up. if it does then CloudKit\iCloud Photos\MMCS must be storing file from icloud itself. I don't see any other explanation.

Well thats my 2 pence and my observations, may be wrong but im going to give it a try. Hope this helps someone,

This is ugly, and i can't find an answer online.  I have 200GB in this **** path

RRead my last post. After cleaning my computer I haven't had the problem since. i never did wipe my phone and the problem doesn't seem to have returned as yet.

maybe it just messes itself up and a good clear out sorts it out.