The hard-disc referred to as /dev/sda failed today in a head crash. The disc has been removed and will not be used any more. No user data was stored in this disc.
No action is required on your part.
A "head-crash" is a phenomenon where the floating read-heads in the disc make physical contact with the fast rotating disc platters, inflicting physical damage on them. A head-crash is not recoverable.
For more information, please consult the Wikipedia article "Head crash."