Ever since the days of DOS, every directory has a special folder in it named "nul". You can use this to create a folder that will sync in Linux but not in Windows. This is because the folder can exist on Linux but not on Windows.
The Nul Folder
The nul folder is designed to make it easy to get rid of output. In the days of the command line interface, where everything was text on a screen, you might not want to see some (or any) information from a command. In this case, you could redirect that output to the nul folder. If you moved a file to a nul folder, then it would be deleted. With GUIs and the Trash Can / Recycle Bin, this is not as useful as it once was; it is mostly there for legacy support. That does not mean we cannot find new uses for it.
Note: In Windows, you cannot create a folder with "nul" as the name, but you can create a nul folder on Dropbox's website.
A Linux Only Folder
The nul folder can exist on a Linux system. There is a similar feature in Linux, but it only exists in one place: /dev/null (and is a file rather than a folder). Since nul is a valid folder in this case, and it can exist on the web access, then you can use it to store files. It is impossible for the folder to show up in Windows, but Dropbox will not penalize Linux. Any files stored there will only available via Linux or the web access. It still will affect your quota, but it can save you hard drive space on Windows.
You could have nul as a top level folder and a hierarchy under it. Everything inside it would be organized but only exist in Linux. That includes subfolders and files within them.
Some Non-Linux Uses
Even if you do not use Linux, you could still use this trick for other purposes. On example is akin to a secret folder. The only way to access it would be from the website. From there, files could be copied and moved in and out of the secret folder as needed. The files would never show up on the computer. Another would be as an cloud recycle bin. Moving a file from a normal folder to a nul folder (from the web access) would delete it from the hard drive but not from Dropbox.
A Note of Caution
It is doubtful that this is an intended feature. It is more likely that this is an interoperability quirk. Dropbox has to work with multiple operating systems that work in different ways. Dropbox may alter the methods it employs or a bug may come into play unnoticed. The more information you store in a nul folder, the bigger the risk you take.
Another thing to consider is that this method may work with "con", "aux", "prn", and other device files too. Those are more legacy devices that cannot be folders on a Windows computer. If you want files to be available in Windows and Linux, do not call the folder any reserved name.
(Okay, there really is a way to create these folders, but we do not recommend it.)
What About Macs?
I did not have a Macintosh computer to test this trick on. Since Macs are so similar to Linux, my guess is that you could make a Linux/Mac only folder. In other words, Windows is always the odd man out. Hopefully some of our Mac users will give this a try and let us know.
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