to relieve most fears..... our local TV/vid/sat repair guy has replaced many 40gb (failed) with 'std' 80gb drives, no probs, I would imagine sticking with a similar brand and spec (with larger capacity) you should be o.k, always check the H/D manufactuers web site for spec of the drive you are replacing (more often than not a lot of drives are pysically the same just more capacity!, so you would get a 'X' brand 'XX' model H/D with say 20, 40 and 80G
, I too have read about disc management software to alter parameters of the drive, prolly a DOS proggy setting up the H/D on a PC first, But can confirm 100% this guy does not use any (as he tends to ask my opinion on PC related stuff).
Also, if your drive has failed, check website for warranty info, I got a duff maxtor sky+ H/D replaced under maxtor warranty even though the sky+ was over a year old