Thanks alband, after fighting this for three hours I fixed it as well. But I had to go even deeper than that. And now, a disclaimer:

WARNING! Following the instructions below can result in loss of data! Do this only as a last resort! You've been warned!

The mail app stopped synching automatically a few weeks ago on my computer. Manual sync worked, but took around 3 minutes.

Then I followed the advice from alband. That reduced the sync time from 3 minutes to 1 minute. That is still a lot of time just to check email, so I still found that unacceptable.

Digging deeper I found another folder similar to DataSharing\Storage : c:\Users\Vladimir\AppData\Local\Comms\UnistoreDB . It has a similar layout, similar files, and a lot of write I/O happened when synching:

http://i.imgur.com/MhQ2hWY.png

http://i.imgur.com/TjicZ8t.png

http://i.imgur.com/HGjUkdu.png

After reading another thread about UnistoreDB (http://answers.microsoft.com/en-us/windows/forum/windows_10-other_settings/unistack-service-constantly-at-100-disk-usage/15b84280-05a4-4440-b97a-77018ecd9fff ) I tried the same thing, to delete the folder, but couldn't because a group of services was keeping it open, Unistack services.

There seems to be a group of such services for each windows account. They don't show in the services MMC, but they show in task manager. You can't stop them, and if you kill the host process, svchost.exe, it starts again automatically, so I had to fight a bit to delete the files.

In any case, after deleting the files the mail and calendar apps seem to have lost their configuration, and showed no accounts. Rebooting didn't help. Deleting the entire c:\Users\Vladimir\AppData\Local\Comms didn't help. Even worse, I couldn't add my accounts back, I received error 8000ffff . The original thread warned about this issue with breaking mail and calendar, but I didn't notice it. Furthermore, OneDrive sync also stopped working.

After reading some more I determined that the c:\Users\Vladimir\AppData\Local\Comms is created only when logging in to a Microsoft account. So I switched my windows account to a local account, deleted the now broken Comms folder, and switched back my windows account to a Microsoft account.

That created the Comms folder again, and OneDrive and the Mail and Calendar apps started working again. I still had to add my mail accounts back and reconfigure everything in the mail app.

The good news: Now the mail syncs in under 10 seconds.

So not sure what's going on with those folders, but it seems over time they get corrupted or bloated/fragmented or something. Apparently the folders contain "Extensible Storage Engine" databases. There is a tool to manage them, esentutl, and it has a defragmentation option. I may try that in the future if I get this issue again instead of deleting them.

One contributing factor to this might be that I have a lot of files in my OneDrive account (18k files). In any case, this needs to be fixed. I hadn't experienced anything similar since Windows Vista that was indexing my disk for days.