Solved this. Just needed to make a script that unmounts the hard drive, tells it to stop spinning and then suspends the usb-port the hard drive is plugged in (because for some reason the usb-drive starts again after about a second) on suspend. And of course the same in reverse order when the laptop resumes from suspend.
In case someone else needs this, here's the script to spin down the hard drive:
And here's the script to resume my hard drive when the laptop wakes up again:
umount -a /dev/your_hard_drive_here
sg_start --stop /dev/your_hard_drive_here && echo suspend > /sys/bus/usb/devices/the_usb_where_hdd_is_plugged_in_here/power/level
echo on > /sys/bus/usb/devices/the_usb_where_hdd_is_plugged_in_here/power/level && sg_start --start /dev/your_hard_drive_here && mount /dev/your_hard_drive_here /mount_point_here