The flush option flushes any outstanding writes from NVRAM to the disk drive.
If you get an error while flushing data, you must purge the data. Purging data "throws away" any outstanding writes in NVRAM.
Purging fast write data should be performed with caution, and only when a drive has failed, as it could result in the loss of data.
If the NVRAM battery is low, missing, or has failed, the menus will be disabled, because the NVRAM is non-functional and data there would be lost.