AIUI, setting or disabling the inactivity timeout in Control Panel only determines how the OS deals with the ATA Standby or Standby Immediate commands. In addition to this, the drive has its own internal APM (Advanced Power Management) which causes it to spin down after a preprogrammed period of inactivity. You may be able to use utilities such as hdparm or HDDScan to set the APM value. Otherwise, some USB-SATA/PATA bridge ICs will send their own preconfigured APM values to the drive immediately after power on. I expect that Seagate Manager software controls this behaviour, but I'm not sure.