USB 2.0 External WD 2TB Elements HD.
Connected to Windows XP SP3 machine, write caching disabled by default. Worked fine.
Since it is intended to be a permanent fixture (never removed), and on a UPS (so power outage is not a worry), I decided to switch on write caching in Windows, to improve performance.
At first, I noticed that BURST write speed did indeed increase. However, sustained writes for more than a minute or so caused massive slowdowns in the entire system, and would cause the drive to almost lock up, causing any other I/O operations to fail. After a few minutes it would return to normal.
Disabled write caching, rebooted, and voila, everything is back to normal. What gives?
EDIT: Tested with a different WD internal drive connected via a 3rd-party IDE-to-USB cable, same results. Going to dig up an old Seagate and test it as well...