OneDrive - Keep on this device

"Always keep on this device" (Files On-Demand) works per file/folder and per device, and large trees can end up only partially downloaded if:

• Some items fail to sync (path too long, size too large, invalid names, admin blocks, etc.).
• Storage on the local drive is insufficient during download.
• Files are still online-only or in a pending state when the operation is interrupted.

From the available information, the most likely causes for only partial subfolders being kept are:

1. Path length / file name restrictions
   • OneDrive enforces a maximum path length (example: fewer than 260 characters for some scenarios) and disallows certain characters in file names. Items that violate these rules will not sync locally even if the parent folder is marked "Always keep on this device".
   • Check for very deep folder structures or long file names and shorten them where needed.
   • Also check for invalid characters (\ : / * ? < > " | or leading spaces) and rename those files/folders.
2. Very large or unsupported files
   • Files over the maximum supported size (for example, over 250 GB in some backup/sync scenarios) will not sync.
   • Identify any very large files in the 260 GB tree and move them out of the OneDrive scope or reduce their size.
3. Insufficient local disk space
   • Even though Files On-Demand is designed to save space, marking a large folder as "Always keep on this device" requires enough free space to store all its contents locally.
   • If the local drive runs low during the download, some subfolders may remain online-only.
   • Free additional space on the local drive, then re-apply "Always keep on this device" to the parent folder or to missing subfolders.
4. Per-device Files On-Demand behavior
   • Files On-Demand settings and statuses are unique per device. A folder fully available on one PC may still be partially online-only on another.
   • Confirm that the device where the issue occurs has Files On-Demand enabled and that the sync client is signed in and running.
5. Items blocked or stuck in sync
   • Some file types may be blocked by policy (especially with work/school accounts) and will never be fully kept locally.
   • If sync is stuck (for example, showing "Processing changes" or pending icons), resolve those sync issues first, then try again.

Concrete steps to move forward:

1. In File Explorer, locate the 260 GB parent folder under OneDrive.
2. Look for any items with error icons or that remain with the blue cloud icon after some time.
3. For those items:
   • Shorten long paths and file names.
   • Remove invalid characters from names.
   • Move or remove files that exceed supported size limits.
4. Ensure there is sufficient free space on the local drive to hold the entire folder.
5. Right-click the parent folder (or missing subfolders) and select Always keep on this device again.
6. Monitor the sync status via the OneDrive cloud icon until all items show as locally available (green check) instead of online-only (blue cloud).

If the folder is extremely large or complex, it may be more reliable to:

• Apply Always keep on this device to critical subfolders in stages instead of the entire 260 GB tree at once.
• Use Free up space on non-essential areas to ensure enough room for the important data.

References:

• Save disk space with OneDrive Files On-Demand for Windows
• Sync files with OneDrive Files on Demand
• Back up your folders with OneDrive
• Upload files and folders to a library
• What do the OneDrive icons mean?
• I am not able to copy a very large folder from the OneDrive folder on my local C: to an external USB hard drive - Microsoft Q&A
• One Drive keeps a huge file in my library on MacOS and I'm running out of space - Microsoft Q&A