24x7x365 here except for the occasional reboots, long power failures or vacations.
 
I really believe heat is the #1 killer of drives. Once I started making sure the drives were not getting warm I have had a lot less failures. I usually outgrow drives now before they fail.
 
Just my $.02

I leave all my drives (internal and external) on all the time. However, that is because I use them every day. If I go on vacation for a week, I'd turn them off. If I were away for a day or two, I'd leave them on.
 
That is for my desktop DAW. For my laptop and living room computer, I turn them on or off as needed.
 
So I guess, for me, it depends on how frequently they get used.
 

mettelus
This day in age, the startup cycle is not as aggressive as it use to be. TBH, just letting Windows power up and power down that drive "as it is needed" is the easiest method - basically will lower power consumption and unnecessary heat over time. So yes, you are thinking too much into it.
 
[aside to clarify some of the above comments]
HDD media is essentially an optical flat (they are that smooth), and coated with a thin lubricant in case the head makes contact with the platter (media). The heads are cut (with an ion beam) with an "air bearing" beneath them so they actually fly 1 microinch or so off the platter (the platter pulls air under the head and gives it flight).
 
In the old days, heads actually came to rest on the media (in a "landing zone"). This zone was pitted with a laser so it was not smooth to prevent the heads from sticking (stiction) to the disk. The fact that a lubricate was used made the risk of stiction greater for two reasons... the head slowly accumulated this lubricant over time, and stiction can literally tear the head off the arm (HGA, or Head Gimbal Assembly). So, in the old days, if you had a flaky drive... back it up immediately, since parking the heads runs the risk of yanking them off.
 
Nowadays, the heads are parked off the media, which is essentially a ramp under the HGA arm to lift them clear. Even in the case of a "loss of power" a capacitor is used to give enough EMF (electromotive force) to park the heads as the capacitor discharges. When powered on, the disk is brought up to speed so that the "air bearing" will have flight speed before the heads are unparked.
 
So, in the past, start/stops were detrimental, but today it is not the case.
 
As far as "misalignment," this is not the case. A head only knows where it is by passing over the media (the head itself only knows "on track" not its own location, per se). Each sector actually has a series of bits prior to the "write zone." One set is left of track, followed by a set right of track, then an "on track" set. Only after it passes over these (and knows it is on track) does the head perform a write or read operation in the data sector. The sector markers are actually written to the platter by a separate machine prior to manufacture of the drive (the heads in the drive itself never write to these), so the machine that does the track bits does, in fact, need to know "where it is" but the heads themselves rely on that data already being present.
[/aside]

Thanks.  Thinking more about it, it's no different than the drives inside the machine other than being newer.  That and it's sitting vertical, which I think would help with heat.
 
Anyway ... thanks to everybody that responded.

Even if it's left 'on', the drive will still spin up and down, which is what will happen when you power it on and off in order to make use of it.  So, overall, not a whole lot of difference as long as the actual use of the drive doesn't change.
 
If you have a Bloody Daft Firmware (TM) NAS box, for example the Seagate Central, then you might want to pull the plug on this if not in use, because the stupid thing responds to all manner of rubbish (e.g. uPNP network traffic from the router) as a cue to spin up and so wastes electricity and reduces its likely operational lifetime needlessly.