Don't worry about it. You should really make sure you are grounded to avoid possible damage to the insides of your computer. I work in the IT industry and regularly (Every day 4 - 5 times) work on the inside of computers without taking any special steps to ensure I am grounded. I have yet to cause any damge through this.
I very much doubt that it mentions anything about working on the hard drive!
I always leave the machine plugged in and rest my forearms on the case, while working on machines. Been doing it that way since 1980 and never had a problem.