I have a stack of old & new PATA/SATA drives with varying degrees of usage--internal & within external enclosures. Rarely used drives in my experience typically have a shorter life as the lube inside the drive can gel up in colder areas(areas with variable temps-cool in the morning, warm in the afternoon) which can result in stictation over time. In the last two years I had to RMA two SATA 500GB Western Digital drives, one which had ~1 hour of usage per month suffered stictation and the other with 5 hours of run time per month became noisy both had 8-12 months of usage. Cases with fans help with toasty drives, 2-3TB drives aren't the coolest running at the moment--I'd wait until 2-3TB drives become mainstream as materials/production improve. (been stung by Seagate on their early 1TB drives)
Leaving a drive 24/7 on a desktop/external non-green drive is perfectly safe, in my experience the reliability is much better as they don't spin up/spin down like the "Green" drives. An idle drive running is less likely to do excessive spin up/down, you can easily expect 4-5 years at 24/7 usage. WD Greens in my experience fail within 2-3 years, they're blacklisted for personal and work use.
As far as Western Digital My Book external drives, they're junk in my opinion as most models lack enough ventilation & cooling(fan). I've seen a fair number of them dropping dead(enclosure chipset bridgeboard) and about 10% of them had a true HDD failure(cooked). I wouldn't recommend leaving it idle if you aren't backing up/accessing files, they can cook to death during the summer/humid conditions.
If anybody wants to see my drive failure database I'll upload it onto Google Documents and post a link. I would note, I mainly use Western Digital, Samsung and Seagate
