An often overlooked feature in 2.4.23+ and 2.6.6+ Linux kernels is the laptop-mode. It may be activated by writing a "5" into /proc/sys/vm/laptop_mode. Laptop-mode, when configured correctly, can make the kernel buffer disk activities for quite a long time and keep the harddisk spun down for most of the time to save power.
There is also a set of userland tools made to automatically manage all aspects of laptop-mode configuration according to the actual mode of operation (ac/battery-status). It is called laptop-mode-tools and you can install it in debian via apt-get or download it from here.
Almost anything you need to know about laptop-mode can be read in your Linux kernel documentation at /usr/src/linux/Documentation/laptop-mode.txt
If you can not find this file on your laptop, then go and get yourself the latest kernel sources.
Keeping the disk in the stand-by state
Laptop-mode is only effective if one avoids spinning up the disk for frivoulous reasons. What will spin a disk up is dependent on the HD firmware, but unfortunately most will spin up on just about every command. To add insult to injury, ATA/SATA pass-through commands are not logged through the /proc/sys/vm/block_dump facility, and thus invisible to tools like lm-profiler.
In systems where the HD spins up too easily, the "hddtemp" daemon will wake up the disk every minute, and must thus be disabled for power management to be effective. The "smartd" daemon is also an offender, but fortunately its default configuration issues commands to the HDs only every 30 minutes.