I am too having the same problem with my model no. WDBAAA2500ABK-PESN purchased in April this year. My all USB ports work when plugged with other pen drives. Even this external hard drive works when connected to other pc. Yesterday I emailed to WDC support and today I was emailed a set of questions which I again answered and replied back.
I have tried on 2 x xp boxes and a Vista box with same problem , installs drivers all ok , disk management sees and shows partitions but options are greyed out ie format , change drive letter etc. Doeds not show up in My computer list either. Any suggestions would be appreciated. WDBAAA3200ABK