What's causing Windows Server 2016 Essentials to lose big chunk of space everyday?

I have a new Windows Server 2016 Essentials with Windows Server Backup and C: and D: drives are configured for Shadow Copies. Total size of hard drive is 2 TB. We are using about 700 GB.
Everyday it keeps losing big chunks of space. After a week I will only have about 90 GB left. A reboot clears everything out and it'll go back to 700 GB. But after another week, it will be the same issue, about 90 GB free space left.
I couldn't find the data / logs that's being used. It is not showing in Windirstat. It only shows 700 GB in total. Please assist on what things I should try to address the issue. Thanks.
Euan VatcherPrincipalAsked:
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what is shown in disk manager?
John TsioumprisSoftware & Systems EngineerCommented:
Get TreeSize Free and set it to run as Administrator...in a few minutes you will see the BIG picture

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binary manCommented:
Directory Report is another tool to run as Administrator.  Should show where all the space is being allocated
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Windirstat run as admin should work fine too
Euan VatcherPrincipalAuthor Commented:
Unfortunately, Windirstat does not show all. It only shows 700 GB and the rest of the storage used is not accounted for. Since posting the question yesterday, I already lost about 20 GB.

However, TreeSize Free was able to find the culprit!
The File History was turned on on one of the client's computer and the server is backing up the File History of that computer D: drive of the server. User has a PST file (2.1GB) that keeps getting snapshot every hour.
John TsioumprisSoftware & Systems EngineerCommented:
The last time i had this kind of issue was on my pure little 128 GB SSD....i was left on a few GB while i was trying to keep everything neatly and tide....well some Sync Option was eating more than 30GB hidden deep into Windows...but for Treesize is a piece of cake those space eaters...
Good to know you found it - don't forget to close this Q
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