Just recently purchased a 3.5" external Western Digital Elements 1.5TB (Caviar Green) drive for $88. I thought it was a great deal for the GB/$, but I'm unsure of whether bigger drives are less reliable (more prone to fail) than their smaller 500GB or 640GB counterparts?
I know about the more platters and parts in a bigger drive and wonder if that is an issue? I've never used a drive above 160GB, so I don't really have experience with such large capacity drive, even though I have looked at reviews on Newegg and Amazon for similar drives.
I plan to use this drive for primarily storage, and I currently have a different, smaller external drive that I use for storage and probably boot up only once a week or so (for a few hours). This is probably the usage pattern I'm going to take with this 1.5TB drive. I'm planning on using the drive as a back up for the information stored on my computer hard drives and also store some data I have that I wouldn't want to lose, but isn't necessarily essential.
More aboutlarge hard drive reliability
The drives capacity has nothing to do with how long the drive will last, and just because the drive has more capacity does not mean that it has more moving parts.
I haven't seen anything to suggest that the large TB+ drives are any more prone to failure than older, smaller drives.