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.
In the Windows 10 Fall Creators Update, OneDrive will change yet again. “OneDrive Files On-Demand” restores the old “placeholder” concept from Windows 8.1, but with improved handling of placeholder files and tweaked Windows Explorer integration that makes it clearer what’s going on. Because downloading and opening those placeholder files is now handled by the filesystem driver rather than a shell extension, you should no longer run into problems where apps try to open the placeholders instead of the actual files—any app, including the command line, will be able to trigger a file download, making the experience more seamless and reliable.
In Microsoft’s demo screenshots (which of course are subject to change between now and when the Fall Creators Update is released), OneDrive placeholder files show up in Windows Explorer with little blue cloud icons next to them. Open a file, and that icon will change to a green checkmark to indicate that it has been downloaded and synced; you can also right-click individual files and folders to choose to sync them permanently, handy for when you know you’ll want to work with files but you’re not sure you’ll have a reliable connection (as on an airplane).
For home users, the key advantage of using placeholder files rather than syncing everything is primarily that it saves disk space. But OneDrive for Business and Sharepoint users will also benefit—using these placeholder files can cut down on the amount of Internet and network bandwidth used to keep files synced as multiple users make changes. Now, you only have these changes synced if you’ve actually chosen to download the file.
As part of the Files On-Demand feature set, Microsoft will also be allowing OneDrive users on iOS and Android to save files for offline use; users of the Android app can use the feature now, while it will come to the iOS version “in the next few months.” The OneDrive for iOS app is also getting an iMessage app that lets you share OneDrive files without leaving the iMessage app; that feature should be available now.