You might also try resetting your Mac's PRAM and NVRAM.

Startup disk (among other things) is stored in PRAM, so zapping it and resetting your startup disk as per ganbustein may help.

(PS: Your Macbook does not have a PRAM battery.)

The new Great Equalizer is the SEND button.

In Memory of Harv: Those who can make you believe absurdities can make you commit atrocities. ~Voltaire