Disk goes to sleep and is never awakened

[JD]

Hi,
I have 3 external drives.
1.5TB, 3TB, and 500GB.
All are seagate drives.
the 500GB is the one that once it goes
to sleep, it is never awakened, and thus
I am unable to even read it after it goes to
sleep. Anything I do to it's mount point,
such as ls, or cat , ...etc
yields io error.
So, Ican only unmount it,
power it down, back up, and remount it in
order to read/write it again.

This does not happen to the other 2 external drives.

I have tried to use hdparm, to  no avail:
# hdparm -B 254 /dev/sdd

/dev/sdd:
  setting Advanced Power Management level to 0xfe (254)
  HDIO_DRIVE_CMD failed: Input/output error
  APM_level    = not supported

# hdparm -B 255 /dev/sdd

/dev/sdd:
  setting Advanced Power Management level to disabled
  HDIO_DRIVE_CMD failed: Input/output error
  APM_level    = not supported

# hdparm -s 0 /dev/sdd

/dev/sdd:
  spin-up: setting power-up in standby to 0 (off)
  HDIO_DRIVE_CMD(powerup_in_standby) failed: Input/output error

So, if the drive does not support APM (advanced power management),
then what is making it go to sleep?