Where Microsoft 365 storage hides: a tenant admin's field guide

The Microsoft 365 admin centre storage report tells you the total. It does not tell you where the storage actually is. Here are nine places storage accumulates on a tenant that do not surface in the top-level view — and where to look in the admin centre to find each one.

1. Preservation hold libraries (eDiscovery holds)

Every SharePoint site under a retention policy or eDiscovery hold has a hidden library called "Preservation Hold Library" that stores anything users thought they had deleted. This library is invisible in the SharePoint UI but counts against the site's storage. On tenants with active holds in place, it can be larger than the live site itself.

Where to look: SharePoint admin centre → Sites → [select site] → Storage, or run Get-SPOSite -Identity [URL] -Detailed in SharePoint Online PowerShell.

2. Second-stage recycle bins

Items users "delete" sit in the first-stage recycle bin for 93 days. If they are "deleted from the recycle bin", they move to the second-stage recycle bin for another 93 days. Both bins count against the site quota.

3. Archive mailboxes

Microsoft 365 includes a 100 GB In-Place Archive with E3/E5 plans, auto-expanding to 1.5 TB. The archive is a separate mailbox invisible from the primary mailbox size — you can see it in Exchange admin centre → Recipients → Mailboxes → [select] → Email apps and mobile, and the counter is at Reports → Usage → Mailbox usage.

4. Teams meeting recordings

A one-hour meeting recorded at 720p is ~400 MB. Multiply by the number of recorded meetings in a month and this is the fastest-growing category on most tenants. Private meetings land in the organiser's OneDrive; channel meetings land in the SharePoint site backing the Team.

5. Teams chat files

Files shared in private Teams chats land in the sender's OneDrive under "Microsoft Teams Chat Files". A 500-user tenant typically has 50-200 GB of these.

6. Version history tails

SharePoint libraries default to 500 versions per file. On a 250 MB design file, that is 125 GB of history — invisible in the file size column, visible only when you open the version panel per file.

7. OneDrive files from ex-employees

Microsoft retains leaver OneDrives for 30 days after account deletion. In practice, most tenants have the retention extended indefinitely, or the account was disabled rather than deleted (retention only triggers on delete). Each one still occupies quota and is still cited by Copilot when the manager runs a search.

8. Checkout orphans

When a user checks out a document and then leaves (or forgets), the checkout can orphan the file in a state where it stores both the current version and the checkout copy. Rare but surprisingly common on long-lived design libraries.

9. Yammer / Viva Engage attachments

Yammer attachments live in a hidden SharePoint site that most admins have never visited. On tenants with heavy Yammer use, this can be hundreds of GB.

TSO surfaces each of these pools in a single scan. The output is a ranked list across all nine, so the top of the Optimize list is always the highest-impact action available on your specific tenant.