A word of caution when upgrading the hard drive on a ThinkPad T43, R52, X41, or X41 Tablet.  These systems contain an IDE to Serial ATA bridge that allows IDE hard drives to be used with the Serial ATA controller.  This configuration can cause programs to function incorrectly and/or slowly when using a hard drive/firmware version that is not approved by Lenovo.  An unapproved hard drive/firmware version will generate a POST Error 2010 at system startup. [more]

The Lenovo website on this issue: http://www-307.ibm.com/pc/support/site.wss/document.do?sitestyle=lenovo&lndocid=MIGR-60169

List of approved hard drives/firmware versions: http://www-307.ibm.com/pc/support/site.wss/document.do?lndocid=MIGR-62282#harddrives

In addition, some Hitachi hard drives purchased on the retail market come pre-installed with a firmware version that generates the Error 2010, but also appears to be up-to-date to Lenovo’s firmware update utility.  Specifically, I ran into this issue with the Hitachi TravelStar HTS7210xxG9AT00.   In order to load a firmware version that will not generate the error, you must update the firmware manually.  Instructions can be found here:  http://forum.thinkpads.com/viewtopic.php?t=20858

Another good reference site for this issue: http://www.thinkwiki.org/wiki/Problem_with_non-ThinkPad_hard_disks