What would be a proper way to shutdown a sata disk connect with a usb interface ?

I'm using sata to usb interface to extract data and to work with end user hard 
drives connected to  a laptop.

I'm using that for a periodic offline backups to HDD and an SSD (I know 
unreliable but that is the best I have for now). 

What I do today when I need to shutdown it are the next steps :

unmount everything
sync
sdpram -S 30  /dev/sdX (I'm not sure if does anything honestly)
Wait for ~20 minutes 
physically touch the disk if I feel any movement , if not unplug the power the 
usb cord and then unplug the power plug.

I have both SSD and plain old HDDs plugged this way. 

I'm feeling that I'm working in an unsafe manner, does any of you have a 
better suggestion how to shutdown the devices correctly to prolong the disk 
life ?



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