The Sync Folder Blacklist: Seven Things That Break Dropbox, OneDrive, and iCloud
Photo: cloud sync folder conflict copy notification on laptop screen, via images.pexels.com
Sync folders are wonderful right up until they are not. You drop a folder in, it appears on three devices, and everyone is happy — until you open the same file on two machines, the app writes conflicted copy 2, and your afternoon disappears.
Sync is not backup, and it is definitely not a shared disk. It is a rumor mill: each device tells the others what changed, and everyone guesses at the truth. Some file types lose that guess every single time. Here is the blacklist.
1. Live databases
SQLite files, QuickBooks company files, Access databases, and anything with a .db extension that an app keeps open while you work. Sync clients copy files in chunks and have no idea a transaction is mid-flight, so they happily upload a half-written page. You get a file that opens, looks fine, and is quietly missing two weeks of rows.
Where it belongs: the local drive of the machine that runs the app, with a scheduled backup pointed at a real backup target. If two people need it, it needs a server, not a folder.
2. Virtual machine disks
.vmdk, .qcow2, .vdi, Parallels bundles — virtual disks are enormous and constantly rewritten. Sync tools see a 40GB file change every few seconds and try to version it. Some clients cope by re-uploading the whole thing; others produce a corrupted disk that boots to a black screen. Either way you lose bandwidth and sleep.
Where it belongs: local NVMe storage, snapshotted at the hypervisor level, or excluded from sync by path.
3. Git repositories with a working tree
A .git folder contains thousands of tiny objects plus lock files that flicker in and out of existence. Sync tools interpret that churn as user activity, generate conflicts inside the object store, and hand you a repository that insists it has unmerged changes you never made.
Where it belongs: a remote like GitHub or a self-hosted forge. That is what it is for. If you must sync a repo, sync a bare mirror or a bundle, not the working copy.
4. Password vaults
KeePass .kdbx files, 1Password exports, encrypted containers — these are single files that get rewritten in place on every save. Two devices saving within the same sync window is a recipe for a conflict copy at best and a vault that refuses your master password at worst. Nothing about that outcome is fun.
Where it belongs: the vendor's own sync service, which was designed for it, or a deliberate manual copy routine. Not a general-purpose folder that also holds screenshots.
5. Mail archives and Outlook data files
.pst and .ost files are enormous, locked while Outlook runs, and rewritten constantly. Sync clients will attempt to upload them anyway, fail halfway, retry, and chew through your upload allowance for days.
Where it belongs: on the machine running Outlook, backed up by whatever tool understands mail files. If you are archiving old mail, export to .eml or .mbox and put the archive somewhere quiet.
6. Photo catalogs and editing scratch space
Lightroom catalogs, Capture One sessions, Photoshop scratch disks — the catalog is a live database (see item one) and the scratch space is temporary by definition. Syncing either one gives you duplicate previews, orphaned sidecars, and a catalog that thinks half your photos are offline when they are sitting right there.
Where it belongs: catalog and previews on local storage, originals on a fast external drive or NAS, and exports — only exports — in the sync folder where you actually want them.
7. Anything holding a lock file while it runs
Game saves with active sessions, CAD projects, audio sessions in progress, Docker volumes, note apps that write a .lock sibling. If the app puts a lock file down, it is telling the world the file is in use. Sync tools do not read lock files. They read timestamps.
Where it belongs: local storage while in use, and a deliberate archive copy once the project is closed and cold.
The one-line test
Before you drag a folder into Dropbox or OneDrive, ask a simple question: does the app that owns this file keep it open, keep it locked, or rewrite it constantly while I work? If the answer is yes to any of those, the sync folder is the wrong home. Give it local storage and a real backup, and save your sync quota for the stuff that deserves it — documents, exports, and the photos you would actually miss.
