Comment 31 for bug 1222313

chris, I think you external drive itself is either in standby or sleep mode as per man hdparm:
"...standby (low power mode, drive has spun down), or sleeping (lowest power mode, drive is completely shut down)."

which wouldn't be an issue if it resumed from it's standby/sleep faster. Hence, for now a WORKAROUND of preventing the drive going into standby/sleep would be best, while a quicker resume potential is investigated.

In order to confirm this, first, when the drive seems to be idle, execute the following and post to this report:
sudo hdparm -C /dev/sdXY

where X is the letter of your drive (may be a or b) and Y is the number of your drive (may likely be 0, 1, or 2). Then, execute the following in a terminal and post the results to make use of APM to prevent the drive from going into a low power state:
sudo hdparm -B 255 /dev/sdXY