You can’t change the startup disk to the selected disk? I am booting my macbook through an external usb drive. At system preferences - startup disk when I try to select my internal harddisk it says:

You can’t change the startup disk to the selected disk. The bless tool was unable to set the current boot disk.


What could be the issue with the internal drive or how can I fix it?
The first thing I'd try is resetting the NVRAM:


Shut down your Mac.
Locate the following keys on the keyboard: Command (⌘), Option, P,
and R.  Turn on your Mac.
Press and hold the ⌘+Option+P+R keys immediately after you
hear the startup sound.
Hold these keys until the computer restarts and you hear the startup
sound for a second time.
Release the keys.


...and see if that solves your problem. If it doesn't, I'd start worrying about a hardware fault.