Unmount second internal drive

Yes. You can unmount volumes by selecting the volume and then using Disk Utility's File -> Unmount Volume command.

I don't know if this spins down the drive or not.

You can also drag the drive icon to the trash which will morph into an eject button just like with an optical disk.

DD

I've had two G5s with dual internal SATA hard drives, which were CTO'd from Apple. If you have your System Preferences… > Energy Saver > [Put the hard disk(s) to sleep when possible] box checked, then the second hard drive will spin down when not needed. That does NOT mean it is unmounted, as its icon remains visible to the Finder. As soon as you need it, it will come back to speed and you can proceed. However, those several seconds when it seems that "time stands still" as the beachball spins waiting for the drive are unsettling at first!

Don't bother spinning down the second drive if it gets user regularly, e.g., if you keep frequently accessed programs and documents there. Mine is used to archive my iPhoto library, so I only need it when I go into that library.

I've had two G5s with dual internal SATA hard drives, which were CTO'd from Apple. If you have your System Preferences… > Energy Saver > [Put the hard disk(s) to sleep when possible] box checked, then the second hard drive will spin down when not needed. That does NOT mean it is unmounted, as its icon remains visible to the Finder. As soon as you need it, it will come back to speed and you can proceed. However, those several seconds when it seems that "time stands still" as the beachball spins waiting for the drive are unsettling at first!

Don't bother spinning down the second drive if it gets user regularly, e.g., if you keep frequently accessed programs and documents there. Mine is used to archive my iPhoto library, so I only need it when I go into that library.

The difference between using the Finder's eject and Disk Utility's unmount on a volume is that eject will unmount all mounted partitions of a disk while unmount will only unmount the selected partition and leave the other partitions mounted.

A good thing to know if one's drive is partitioned....