External hard drives which connect via USB often do not offer a standard way to access S.M.A.R.T. to assess their health. Fortunately, some USB to ATA bridge chips at least have a proprietary method that can work, and smartmontools just got support for some common chips. The support came after the 5.38 release which is currently available, so I got smartmontools source via SVN and compiled for mingw32 from cygwin. Then I ran smartctl as administrator, and used the "-d usbcypress" option. I was able to get S.M.A.R.T info from my Archos Jukebox Recorder v2, which uses an ISD300A1 and is doing fine.
For more info see USB device information and the list of supported devices on the smartmontools wiki.