Eventually the vibes from the bike will kill the HDD (after many many hours though most likely, unless your unlucky). Best to store in a rucksack or similar.
SD Versions work much better. also you get what you pay for in cams.
I have a sony system recording to SD Card which allows recording from 2 bullet cams providing (front And Rear picture in picture system) 
SD is a form of solidstate. Basically memory card or 'SSD HDD'.
So SD, MicroSD, Compact Flash, TransFlash, xD or Sony's wierd memory card as used by the PSP and what ever other types there are. All generally (Class5 or up) also have a faster bitrate than spinning HDD's as they can shift data quicker than the needle can edit its sectors.
Plus they are also generally smaller is size and need less power if your pulling from your bikes battery (Dont forget a Regulator if you do try this and a Voltage Rectifier if needed as a lot of Solid state devices run at 5V).
