The ferrofluid itself is contained in a highly engineered clear suspension fluid which keeps it from evaporating, staining the glass or breaking down. If kept around room temperature it will last many years.
Nope. If your hard drive was made after the year 1999 then you are completely safe. The data density for new drives is so dense that even a very strong generalized magnetic field will not erase the date from the drive. You should keep the handheld magnets at least an inch or two away from running hard drives or metal objects for good measure.
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Using a standard CR2032 battery (included) the LED will be very bright for around 80 hours, and then start becoming dimmer until it turns off after 160+ hours. The diode itself is rated for 10,000 hours.