why don't external drives turn off

Hi,

I've tried looking for this topic but can't find it anywhere here.

I have both self powered firewire 800 and external mains powered USB drives connected to my thunderbolt display running off an MBP. When I shut down the MBP, the self (port powered) drives shut off, but the mains powered drives spin indefintely. They don't do this if connected directly to the MBP. Whats up? any workaround? I'm worried that they'll have a shorter life from just running constantly.