windows 7 - What does "Write cache buffer flushing" mean

As in the Windows 7 Disk Policy Options:

[x] Enable Write Caching on the device
[ ] Turn off Windows write-cache buffer flushing on the device 

Is the first the cache on the physical disk, and the second the windows cache?

(I've googled, but didn't find a clear answer).

Answer

See "more information" section from http://support.microsoft.com/kb/332023:

Many disk devices provide enhanced performance through the use of an onboard cache, which provides read-ahead caching for data that is being read from the disk, and write-behind caching (or delayed writes or "lazy" writes) for data that is being written to disk. In some cases, it is important for data to be written to the physical disk immediately, and not retained in the disk's onboard write cache to be written later during an otherwise idle moment. This prevents loss or corruption of this data if the disk or controller (wherever the write cache is implemented) suddenly loses power

Write caching improves disk performance greatly, so leave it on for internal hard drives.

Windows turns write caching Off for flash drives and other usb hard drives by default, normally you do not have to ever modify these settings, Windows takes care of it for you.