Things to try:

Advanced power management mode off (works with newer Seagate drives):

	hdparm -B 255 /dev/sdX

Standby timer disable (may work with Toshiba external drives):

	hdparm -S 0 /dev/sdX

Clear SCSI 'STANDBY' setting in power condition mode page (works with older
Seagate "Backup Plus" enclosures):

	sdparm --clear STANDBY -6 /dev/sdX

Disable wdidle setting (for Western Digital Green internal drives only; be sure
this is appropriate for your drive before you use it, as it can be dangerous):

	hdparm -J 0 /dev/sdX

Failing all else, write a script that touches a file or creates a directory
every n seconds. Helpful hint: mkdir is a synchronous operation on NTFS /
Windows; i.e. if you mkdir, Windows will do the equivalent of sync on it right
away, keeping the drive awake.