What is special about the _Data folder?

I had a folder named 'data' in my Project panel which I renamed to '_data'. All was well. I then renamed it to '_Data' which was refused by the IDE (which I assumed was because it was only a case change and Windows can be pernickety about these things). However, when I opened Unity just now, the '_data' folder is missing in the Project panel and any Inspector associations are now broken. Looking in Windows Explorer, I notice that the folder is now named '_Data' but does not appear in Unity.

So, is this a reserved name? This question suggests that [exename]_data is created during compiles. How do I recover cleanly from this? I would ideally not like to recreate all the Inspector associations of files within this folder.

Thanks!