sg_start dose not support some USB External Hard Disk

[nas126mail <nas126mail@126.com>]

Hi: 
    I'm a user of sg3_utils-1.28. 
    When I use  sg_start  to spin down  some USB External Hard Disk , some  does  work and some  does not work , can you give me some advise?

    I have tried  commands listed below,  they all  failed:
        sg_start  --stop    /dev/sda
        sg_start  --pc=2    /dev/sda
        sg_start  --pc=3    /dev/sda
        sg_start  --pc=5    /dev/sda    

    I have tried USB enclosures listed below,  they all failed:
        Brand         / Chipset       
            / Size / Type / Err Msg
        ==============================================================================
        Tekram        / Ali m5621                / 2.5  / IDE  /
        Sonny         / Au8390                     / 2.5  / IDE  /
        Hitachi       / CY7C68300C-56PVXC        / 2.5  / IDE  /
        IBM           / B31-MBL &nb
 sp;                / 2.5  / IDE  /
        Newman        / GL811E                   / 2.5  / IDE  / invalid field in cdb
        GigaSave      / GL850 0452MKOY1-16       / 3.5  / IDE  /
        Keshuo        / JM20339                  / 3.5  / SATA /

    Thank you for your help!