Mac – Small external hard drive in time machine

Make sure the drive is from a reputable supplier. I've used Western Digital external drives without a problem.

When you get the drive it will most likely be formatted in NTFS format (for Windows), so when you plug it in, you will need to open Disk Utility and repartition it as HFS+

A Firewire 800 drive is fastest, although since the backups are incremental and happen in the background (with only the changed bits/files going across), I believe the cost/benefit factor is weighted towards getting a cheaper but higher capacity USB 2.0 drive. That's just my opinion.

That last paragraph is very difficult to understand, but if I follow, creating a RAM disk is not going to help...primarily because there's no problem to fix in the first place; Your concerns are unwarranted.

Time Capsule hard drives in particular are very low failure rate, and above that, Time Machine does not tax your system indefinitely. When the timer starts a new backup, the hard drive will spin up. On completion, it will spin down as necessary. These drives, like most of decent quality today have a high MTBF & MTTF, so relax.