Hard drives suck. Large drives doubly so. Keep regular backups.
As for the other question, seek times and capacity are largely independent. Seek times consist of three factors: the rotational latency (which is determined by the rotation speed), the track seek time (which is determined by how quickly the arm can move and is somewhat dependent on the diameter of the drive plater), and the settle time.
Of these, the speed of the drive rotation isn't tied to capacity at all. They give that in RPM as one of the major specs of a hard drive, e.g. 5400 RPM, 7200 RPM....
Similarly, the speed at which the head arm moves is not capacity-dependent; platters on modern HDs are pretty much all either 1", 1.8", 2.5", or 3.5", and drives with one platter size aren't interchangeable with drives of a different size without serious adapters, so you can pretty much ignore this factor as well. There are differences in the seek speed of different mechanisms, sure, but they aren't related to the capacity in any real way.
This leaves settle time. For short seeks, this dominates. You're mostly doing short seeks. Therefore, this is at least a bit important.
Higher capacity is achieved either by adding more platters or by increasing the density.
Increasing the number of platters should have no real impact on settle time whatsoever, though it may make overall performance faster to some degree if the fractional-track seeks are exceptionally fast (which is usually the case) as it means that there is more data available without doing a seek.
Increased density may increase the settle time a tiny bit, but overall, generally improves performance, again, by reducing the average number of seeks needed to read a given amount of near-contiguous data.
The thing is, the density-caused settle penalty is basically lost in the noise---it is likely to be severely overshadowed by the extra throughput gained from the density increase that necessitates the more precise head placement.
In short, I wouldn't worry about the seek performance. I'd worry instead about how you're going to back up a drive that big.
