unicorn and external disks can use suspend.
You should be able to do this by suspending the controller the disk is attached to. If possible, move the disk to its own controller.
For example:
$ fgrep Maxtor /var/run/dmesg.boot
umass0: Maxtor (0x0d49) Basics Desktop (0x7410), rev 2.00/1.25, addr 1
sd0 at scsibus0 target 0 lun 0: <Maxtor, Basics Desktop, 0125> disk fixed
$
Now locate the controller sd0 is attached to:
$ drvctl -lt
[...]
xhci0
usb1
uhub1
umass0
scsibus0
sd0
[...]
$
So, that's xhci0 (other USB-controller names look like: ehci0, ehci1, ohci0, etc.).
Suspend it (this will suspend all devices connected to that controller!):
$ drvctl -S xhci0
Resume with:
$ drvctl -R xhci0
Note that a resume might not bring back all the devices. When I tried this just now, the disk and the USB-mouse came back up, but my USB-keyboard did not (even after unplugging and re-plugging!).
