Last I heard, drives would physically fit but not work.
You could potentially turn to flash drives if you're willing to install linux. Many of the high speed ones (available in sizes up to 8GB and 16GB) are faster than the internal drive, with the caveat that they're external and the usb thumb drive form factor sucks for transportation unless removed. You could also use a normal external harddrive. Linux is capable of installing to an external drive (and the latest versions of linux even support the tablet pen, though I'm not aware of any writing recognition software, though you can probably download some).
Alternatively, windows vista can take advantage of a USB drive for its readyboost feature which gives some performance boost, but I found vista was unbearable slow on the x41t even with readyboost. (the internal sd slot will also work for readyboost)