I am thinking of buying the HDD adaptor for my T410 so that I can place a 2nd HDD in the optical drive and I'm planning on putting in 1Tb drive as I need a lot of storage space. My question is, how much battery life will be affected if I go that direction - having 2 HDD's drawing on the system battery?
Generally it will add as much additional current draw as the current HDD inside. This will result in an impact (loss) of about 30-60 mins battery life - depending on usage of that HDD.
7200 rpm HDDs drain more and often louder and may run warmer than a 5400 rpm drive, so there may be some trade offs. A 7200 rpm drive will be faster, but will decrease the total run time, from both the standpoint of power it draws and the additional heat generated that must be cooled, which could result in the system fan running at higher rpm and more frequently which would consume a bit more of the battery.