Then your hub is not doing a good job of providing the 500 mA that it supposed to give to devices, or your USB powered hard drive actually requires more than 500 mA to do its thing.
Make sure the power brick that your powered hub came with is capable of powering the N+ devices you have at the same time. Most of the 4 port hubs come with 2A power supplies, however sometimes they skimp on actually being able to deliver that power.
