Windows 10 was a big improvement over Windows 8.1 in most important ways, but it made a big change to the way OneDrive syncing worked.
In Windows 8.1, you could see all the files you had stored in OneDrive, but the operating system would only actually download and open the file when you needed to open it.
At least for PCs that usually have Internet connections, this was a neat way to offer cloud file syncing without consuming gigabytes of space for infrequently used files on every computer you were signed into.
But the behavior could be error-prone—apps could attempt to open the placeholder files created by OneDrive rather than the files themselves—and it could create confusion about which files were actually available offline.
So in the initial releases of Windows 10, Microsoft changed the behavior to be more Dropbox-esque.
All OneDrive files are now downloaded to your PC when you sign in, though as with Dropbox you can choose to only sync selected folders based on what you need to have available at all times.
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