To be used as a back-up medium I want to install a second HD in my pc with
Windows 98se.
To save on energy, but especially to keep the noiselevel as low as possible,
I want to switch this drive off when it is not in use.
How could this be done? Could it also be done without restarting the
computer?
I will be grateful for any assistance.

Unfortunatley there is no way that you can turn off a single HDD but
you can set the power managment system so that all of the HDD's go off
after a set period.

Sorry about that. (trust me i am an IT Professional)