ESRI: Registration Failed, Could not write to disk.

ESRI messages can be confusing sometimes.

Why it happens?

Chances that you are here because you encountered this error and you don't know how to fix it. Well the fix is easy but ESRI like to waste your time instead of displaying a little more informative message it just slam whatever the OS returns right back at you.

Anyways, this error usually appear because you were trying to install a new extension on your ArcGIS desktop environment (I assume 10 and above). It ESRIRegAsm.exe to register their dlls, whatever launches this executable must have Administrator privileges because 

The Fix

Run as Administrator. 

If you are running ESRIRegAsm.exe manually, run the Command prompt (cmd) as administrator.

If this error pops up because you installed some exe setup file, then run this setup file as administrator. 

If you are running an MSI file (Windows Installer, they look like this ) , for some reason you cannot right click and run as administrator, so you can use this method.

P.S.: Yes, you have to run as administrator even if you are an administrator on that machine, yeah, don't ask.