for my PC laptop for years I used 3.5" seagate drives in external enclosures with external power -- very noisy but ok for desktop backups
for portable access I has two 2.5" drives I used -- USB powered and all you could here were the clicks as they were accessing data
when I was in the UK I got SWMBO a 750GB Maxtor (it has seagate inside) OneTouch
It had external power but beautifully silent
I bought another Maxtor (1TB) as a Christmas gift for my second son - also still soundless
When I bought my Macbook I got a LaCie USB recommended by the Apple store person -- this I carry as I travel
And then I have bought two 1TB Maxtor Onetouches for myself (duplicate backups)
Even though the Onetouches are more expensive than other Maxtors I am buying them for consistency of appearance and because I know how quiet they are
On hardware forums lots of people decry the Seagate drives - but after some 6 years of usage I still have had no problems with their drives
First B&O (1976) was a Beogram 1500 ... latest (2011) change has been to couple the BL11 with the BL6Ks *sounds superb*