I don't see any problem with PATA -- They are just as reliable, just as fast ... Now I will agree that perpendicular recording will speed things up, slightly and newer drives tend to be slighly faster than older drives (regardless of interface) but the gains are really not signifigent.
When replacing 45TB, there really is nothing to do but bite the bullet: It's gonna cost. Unless, they all have to be identical because of RAID requirements or some such issue, then my philosophy would be buy when they're cheap and do it over time.
I really don't see any advantage of Hitachi over Seagate. Matter of fact, I really try to avoid Hitachi (mainly because they tend to be hotter) like WD or Maxtor (These brands just tend to be junk). My understanding of Samsung is that as size goes up, reliability goes down with the optimum being 160GB. Their 250+ GB drives just aren't as good. As a brand, I rather like Seagate drives...
Me, I'm replacing my 250's with 400-500's; my 120's with the 250's; my 80's with my 120's; my 45's with my 80's and junking anything that I don't like along the way. Not everything is being replaced to gain size: Often I'm combining multiple drives into a single larger drive. If for no other reason, than getting rid of a controller or to improve cooling: It's all good stuff.
I am also carefully looking into replacing a couple of my 2500+ XP Bartons, that have become somewhat unreliable, with either dual core dual Opterons (F socket) or a couple dual core Woodcrest Xeon's. I'm eagerly antisipating the results of that particular race which should play out later in the year. Just replacing two machines should double my folding output...
