I think your HDD is powered solely by TV, and USB port is being left powered-up when the TV is in standby mode.

If you have Seagate HDD than you can change settings with Seagate Manager or Seagate Dashboard software which came with your HDD.

For Seagate Expansion HDD download SeagateDriveSettings.exe at their site.

For other HDD brand's look at their site's.