Safely power down an individual disk drive (non-SCSI)?

[Modulok]

OBJECTIVE: Safely power down an individual disk drive (SATA, PATA, not
SCSI). Is this possible? Whether for physical replacement or for
saving power, it would be nice:

1. Issue a command to safely power down an individual disk drive.
2. Physically remove the disk and replace it with a new one (or not).
3. Issue a command to power up the disk.

I could get really carried away: Need to update system memory? Issue a
command to migrate all pages to a specified memory module and power
down the one to be replaced, plug in the new module in and issue the
command to bring it online...rinse and repeat. Fans, memory modules,
disk drives, processors (in a multi-processor system of course).
Granted, this would likely require some fundamental hardware changes
and cost developers a few migraines...I can dream.

Anyway, powering down a disk drive?
-Modulok-