SO. you are saying that you entered that command into a shell prompt ?
Any changes you make to hdparm are reset to default on re-boot.
if you entered that command into a bootup config file, like anything in /etc/rc.d/rc.local then simply remove the line and drive will return to stay default dureing boot.
HOWEVER... if you didnt add that line to a bootup config script, then a reboot would have re-set the disk to defaults, and STILL caused a crash.
i havent looked up exactly what you have done, but it seems to me, that you made a bad change to hdparm, which damaged some files.
i think you need to re-install
ask for a second opinion, im no guru.