you can keep your hard drive on indefinantley if you like . the onley thing that can possibuley wear out is the bearings, even then bearings in HDDs are fluid dynamic bearings and wont wear out . older drives have ball bearings but they last extremeley long too.

thers nothing wrong with leaving it on all the time. there are some HDDs that have been on 24/7/365 for 20 years and are still running fine.

if it works the way i think it dose it should turn itself off when you turn the computer off , or go into some kind of stand by mode where its on but the disk itself is not sitting ther spinning.