WD Reds are optimized for NAS use, and thus expected to run pretty much continuously.
A hard disk cannot start spinning instantly: if a disk was stopped when an I/O request for it arrived, it would incur several thousands of milliseconds of extra latency as the disk would have to spin up before the request could be satisfied.
It is possible to configure the disk to go to power saving mode if it's been idle for a specified amount of time, but this is not the default for WD Reds. (Some WD Green models do that by default - but that is not always a good thing: see the description of the
-J option in the man page of the
You could use the
hdparm commands to intentionally send the disks that are unmounted and you're not going to use into a power saving state, causing them to spin down.
Disks don't know whether they are mounted or not: the disk controller will just send in requests to read or write groups of blocks. After all, you can read or write unmounted disks e.g. using the
Mounting a filesystem in Linux basically means that certain kinds of metadata and caching are maintained in RAM for efficient filesystem access.