Grub update warning in Ubuntu 14.04

You can't use the GRUB_HIDDEN_TIMEOUT setting and the GRUB_TIMEOUT setting at the same time. This is true even when the hidden timeout is set to 0.

You can comment out the line that you don't want.

For example:

#GRUB_HIDDEN_TIMEOUT=0

After saving the change, run sudo update-grub again.

Short answer:

#GRUB_HIDDEN_TIMEOUT=0
#GRUB_HIDDEN_TIMEOUT_QUIET=true
GRUB_TIMEOUT_STYLE=hidden

Or just delete the first two lines in the entry above and replace it with

GRUB_TIMEOUT_STYLE=[menu|countdown|hidden]

Explanation:

As of this time (2016) the GRUB_HIDDEN_TIMEOUT and GRUB_HIDDEN_TIMEOUT_QUIET options have already been deprecated. So don't use them any more. Instead, you can use GRUB_TIMEOUT_STYLE.

According to info -f grub -n 'Simple configuration', you have this:

'GRUB_TIMEOUT_STYLE'

 If this option is unset or set to 'menu', then GRUB will display
 the menu and then wait for the timeout set by 'GRUB_TIMEOUT' to
 expire before booting the default entry.  Pressing a key interrupts
 the timeout.

 If this option is set to 'countdown' or 'hidden', then, before
 displaying the menu, GRUB will wait for the timeout set by
 'GRUB_TIMEOUT' to expire.  If <ESC> is pressed during that time, it
 will display the menu and wait for input.  If a hotkey associated
 with a menu entry is pressed, it will boot the associated menu
 entry immediately.  If the timeout expires before either of these
 happens, it will boot the default entry.  In the 'countdown' case,
 it will show a one-line indication of the remaining time.