Considering the huge number of posts concerning Hard Disks and the related, "beachballing," "stuttering," "pauses," etc. etc., I thought it might help everyone to have ONE thread where people can look for information on which HDs work or do not work. Please include a computer, HD and OS version description, as in my example below. 1. Model Name: MacBook Pro Model Identifier: MacBookPro5,4 Processor Name: Intel Core 2 Duo Processor Speed: 2.53 GHz Boot ROM Version: MBP53.00AC.B03 <--- Firmware ver. 1.7 2. OS 10.6.2 3a. HD: Seagate 7,200 rpm 500GB, Model: ST9500420AS. Verdict= No Problems, does use up the battery a little fast than the stock drive. Good performance, quiet, no perceivable vibration. 3b. Toshiba 250GB 5,400 rpm Model (unknown) stock drive, Verdict= No Problems, but rather slow performance, Toshiba, totally silent to my ears. 3c. Western Digital 5,400 rpm Scorpio Blue, 500GB Verdict= Annoying stuttering, pauses (not long enough to cause the beachball to appear, but bad enough to make the HD totally unusable.