r/o365

Disclosure first because the mods will catch it: I wrote this, MIT licence, link at the bottom, no upsell.

School IT background, but the use case generalises to any small-to-mid-size M365 estate (schools, small businesses, charities, roughly 20-500 users) wanting proper drive-letter mapping for OneDrive and SharePoint where the OneDrive sync client isn't the right shape. Wanted a drive-letter sync setup with a few specific things: image thumbnails that actually render in Explorer for cloud-mounted files (most existing tools don't), accurate cloud-quota numbers in the drive-properties dialog (refreshed from Graph), a local-first design where files always exist as real NTFS first and sync to the cloud in the background (rather than presenting the cloud as a virtual filesystem), one config.json that deploys to the whole estate, and zero telemetry. Didn't find one off the shelf with all of those, and was also curious whether I could build it myself. Side project that turned into a real thing.

It's called OneSync. .NET 8, Dokan for the FS bridge, MSAL + Graph SDK for auth and sync. Self-contained exe, Intune-deployable.

Highlights:

- True drive letters via Dokan (not subst, not "OneDrive sidebar entry under TenantName - LibraryName")
- One config.json deploys to the whole estate. OneSync probes Graph at startup and silently skips drives the signed-in user can't access, so different roles (staff/students, sales/finance/ops, whatever your split is) share one config
- Accurate cloud quota shown in Explorer drive properties (refreshed from Graph)
- Image thumbnails (JPG/PNG/HEIC) render like a local disk
- LRU eviction below a free-space threshold so shared/hot-desk laptops don't fill up over time
- Office desktop AutoSave + co-auth on docx/xlsx/pptx (via file-association redirect to ms-word:ofe with the direct SharePoint URL, slightly cursed but works)
- Zero telemetry. No analytics, no licence-server pings, no anonymous usage stats. Only outbound calls are to graph.microsoft.com and login.microsoftonline.com.

One war story while I'm here: Dokan's FindFiles callback enters synchronously from the kernel, so anything that blocks in there hangs Explorer. The first version called Graph synchronously to enumerate folders via .GetAwaiter().GetResult() (I know, I know). Worked fine until Graph went into 429 cooldown, at which point opening a folder hung Explorer for ten minutes while the cooldown lifted. I learned a lot in those ten minutes about how Explorer's UI thread feels under those conditions. Current behaviour: if Graph is in cooldown, folder browse returns immediately with on-disk placeholders and file hydration kicks off via Task.Run. Obvious in hindsight.

Repo: https://github.com/madeyouclickstudio/OneSync (DEPLOYMENT.md covers silent install switches and Intune detection rules)

Honest caveat: I'm running it with a handful of pilot users, not a full estate. The fleet-scale stuff is engineered for it but not battle-tested. Anyone deploying this to thousands of users is a pioneer at their own risk. Bug reports very welcome.

Yes I know about the OneDrive sync client. We tried it. The reasons it didn't fit (no real drive letters, every library is its own sidebar entry, no way to cap disk usage on shared laptops, no central deployment for which libraries each user sees) are exactly the reasons this exists. If those don't bother your fleet, OneDrive sync client is fine and you should ignore me.

Tell me what's wrong with it.