Spin down secondary HDD on Windows 7 HP Notebook

Hot swapping out the secondary disk might be a solution.

The free utility HotSwap! does that :

It does the same thing as you can remove device from Device Manager but it provides much friendly user interface as you remove the removable device from the "Safely Remove Hardware" icon in the notification area.

It also ensures that all data are written and flushed to the disk before the device to be hot-swapped, so you can use SATA/eSATA drive as a removable device much alike USB/IEEE1394 drive.

Occasionally it will spin down on its own but only after about 10 min of not accessing it, and then it spins back up on its own shortly afterwards for no apparent reason.

You can use Process Monitor to see what I/O events are happening, or DiskMon for pure I/O.

With Process Monitor you could then filter it so the Path starts with your secondary HDD station letter.

Will it even make much of a difference on either account?

The main reason HDDs spin down is to conserve power. I don't think it helps to prolong the life as it might as well just cause more stress on the HDD due to spinning down and up than just letting it run idle. On the other hand, it does save you some wattages and thus battery power. But why estimate when you can measure?