When I bought my Thinkpad X31 (X31 model 2672REU), I ordered it with a 40G drive. I thought I would never ran out of space. Then came iTunes with Apple Lossless encoded music, and my Nikon D70 which generates thousands of pictures at 3 meg each, my 40G drive went down to 1G free for months. After some research, I ordered my hard drive upgrade components from Drive Solutions. They are reasonably priced and shipping was fast. I ordered a Hitachi 100G 7200 RPM Travelstar drive at $205. The 80G drive at $165 is actually better bang for the buck, but I figure I will only upgrade my harddrive once on this laptop, so might as well go for the largest drive. The travelstar is highly rated for speed and quality. I also ordered the EZ Upgrade Universal Hard Drive Upgrade Kit USB 2.0 by Apricorn. This software and case combination lets one clone the existing drive over to the new drive via USB.

I have outline my successful upgrade steps below. I did run into a problem at the beginning, when the EZ-GIG software failed with a read error from my original drive. After some research, I determined that the error is due to some disk corruption/bad sector on the drive. I ran the XP version of "scandisk" and fixed the source drive.

Upgrade Steps

BACKUP your data!!
Run "scandisk" on the source drive. On Windows XP, right click on the drive, select property, tools, Error-checking, check disk options select scan for and attempt recovery, and click Check Now. See this write up from Microsoft.
Run defragmenter. It will speed things up a little.
Put the new drive in the Apricorn upgrade kit's case. Make sure you get the pins lined up and don't bent them.
Power off the laptop.
Attach the USB cable to the upgrade kit containing the new drive. Make sure it is switched on at the kit.
Insert the Apricorn software CD into the CD Drive.
Power on the thinkpad, hold down the F12 key to get to the boot option menu.
Boot from the CD
Following the instructions on the EZ-GIG documentation, but basically:
1. Select clone
2. Select expert mode, because I only want to create a same size hidden drive D for the IBM rapid restore instead of a proportionally sized partition. I don't know if I really need this hidden partition anymore, but just in case.
3. Do the copy, go to lunch or something
Once the copy is successful, shut down everything.
Unplug the USB drive.
Swap the new and the old drive.
The harddrive on the thinkpad X31 is very well placed. You only need to unscrew one screw to slide the drive out. See the IBM/Lenova hardware maintenance guide for more info.
Detach the black flip piece, and unscrew the four screw to release the "cage" for the bare drive.
Attach the "cage" to the new drive, reattach the black flip piece, insert the new harddrive.
Boot up the new machine
XP will notice the new drive, did its own "install", and asked you to reboot.
Reboot
and you are done !