Recently, Microsoft announced they're increasing OneDrive from 1 Terabyte, to unlimited. Unlimited as in infinity, endless, never ending, vast. You get the point. The only catch is that you have to have an Office 365 account. Considering you can get one for as little as $69.99/year, this is a phenomenal deal even if you don't use Office. If you get the 5 user edition for $99.99, you'll get unlimited space for everyone in your household. That's like 5 infinities! Kids know that the more infinities you have, the better.

The killer feature for OneDrive is that the files you store in it don't have to actually take up any space on your hard drive. After a file sits unused for some time, it can switch to an online-only mode. It will still appear to be on your computer, but it takes up zero space. For example, my OneDrive folder has 100+ GB on it, but on my laptop, it only takes up 307 MB on disk. When I try to open a file that's not actually on this computer, it will automatically pull the file down and make it available offline. It's all transparent to the user. This is a big improvement over the selective sync option in Dropbox.

Virtual Drive Space Screenshot

Now for the bad news. On my desktop, I have a 9 TB storage array, and a 512 GB SSD. My photo collection has ballooned in size, and is now well over 200 GB. If I were to simply copy these files into my OneDrive, I would both fill up my SSD, and the files would ultimately end up online-only. I want to have quick (read: local) access to all these files.

My first thought was to use robocopy, a powerful file copy utility included with Windows. My hope was that I could maintain a backup copy in my OneDrive, and simply mirror the new photos as they were added. I tried every combination of command line parameters, but I always ran into errors from robocopy (file cannot be accessed by the system) because it wasn't designed to work with files that only appear to exist.

Then, a breakthrough. I learned that OneDrive supports WebDAV. WebDAV is a protocol that runs on HTTP and allows you to open a remote resource as a folder in Explorer. It let's you do tricks like open a SharePoint document collection in an Explorer window.

Mounting OneDrive as a Folder

First, you'll need a special ID, called a cid to make this work. It's easy to get, just go to, and click Files. In the URL, grab the cid value after the equals sign.


Next, open an explorer window, navigate to your computer ("This PC"), and click "map network drive" in the ribbon.

Map Network Drive

The drive you're mapping is, be sure to enter your CID in that URL.

Map Drive Details

When prompted for a username/password, use your live ID for the username. For the password, you can use your live password if you don't have 2-factor authentication enabled. If you DO have 2-factor authentication, generate an app password here.

Congratulations, you now have OneDrive mapped as a drive on our computer. This is different than the typical OneDrive folder, because changes on this drive are reflected immediately in the cloud.

WebDAV Drive


Now that we have a drive that takes up no space on our computer, but allows us to copy files to it without affecting our local copy, our work is easy.

For my photos folder, I run the following command line script:

Robocopy e:\photography "z:\backup\photography" /m /e /purge /mt

Command Line

Here is an explanation of the parameters I'm using:

  • e:\photography - The source of the files I would like to backup.
  • z:\backup\photography - The destination of the backup in OneDrive.
  • /m - Only copy files that have been modified. The archive bit on the file will indicate this, and will be reset.
  • /e - Include subfolders.
  • /purge - Delete files that have been removed in the source. You may choose to skip this so that accidental deletes won't propagate.
  • /mt - Use multiple threads. This speeds up the upload by a large factor.


Performance of WebDAV is not that great to be honest. Navigating around the drive is an exercise in frustration, but it wasn't designed to be used that way.

Performance of copying the files is alright. I've let it run overnight and it copied up a few gigs. After the initial load, the speed won't be an issue. But truthfully, I don't need it to be fast. It's just an additional backup.


Everyone repeat after me. OneDrive is NOT a backup solution. That being said, it can be part of a balanced backup diet.

You should always have an additional off-site backup, more are better. I rotate external drives for this purpose.

Update 2014-11-07

I found out WebDAV is messing up the destination timestamps, which makes the subsequent backups overwrite files that haven't changed. I've updated the robocopy paramters to use the archive bit on the files. Keep in mind that the archive bit doesn't work if you have multiple backup processes using it.

Update 2014-11-09

Added the /mt flag, which speeds this up 5x-10x.

Update 2015-02-10

A friend suggested an alternative, which is to use SyncBack Pro since it now has native OneDrive integration. The only limitation is that it uses the API and has a 100MB file limit.