A devotee of horses and of horse racing : Granted hardboots can be sentimental about a horse on the lead (1940s+)
A boot which resets the entire system.
The phrase has connations of hostility toward, or frustration with, the computer being booted. For example, "I'll have to hard boot this losing Sun", or "I recommend booting it hard".
Hard boots are often performed with a power cycle.
Contrast soft boot. See also cold boot and reboot