How can I increase the time before my external drive powers down?
Cocktail is an award winning general purpose utility for Mac OS X


This app does exactly what you are after (plus much much more), but it will set you back $15.

Alternatively, the command pmset appears to do this as well, but without the fancy GUI. For example pmset -c disksleep 60 will prevent all disks from spinning down within 60 minutes of activity when plugged in. You can change the -c part to specify a different power condition:


  The -a, -b, -c, -u flags determine whether the settings apply to battery
       ( -b ), charger (wall power) ( -c ), UPS ( -u ) or all ( -a )