OK, if your projecting a scope film onto say a 130" scope screen without an A-lens you're actually projecting a ~138" 16:9 image - (the native aspect ratio of the projector is 16:9 and this doesn't change). Both images are 120" wide. The 130" scope screen is ~50" high. A 138" 16:9 image is ~67" high. The difference in height between 67" - 50" = 17". Half of the 17" is the black bar above the screen and the other half is the black bar below.
When you display a 16:9" image on the same 130" scope screen you want to fill the full 50" height of the screen, but not have it spill over the top and bottom of the screen. So you alter the zoom to reduce the image height. That means the width of the image also reduces - in this case to ~90" to give a 16:9 diagonal image of ~103". You're keeping a Constant Image Height on your screen (ignoring the black bars above/below for scope material).
Effectively you zoom between a 138" 16:9 image (that has black bars above and below the screen) for scope and a 103" 16:9 image for 16:9 material. It's a lot easier to see it demonstrated than read a wall of text.
Sounds good. All drives will fail eventually. There's not a lot of difference between them. Chance of failure is minimal. If it does fail then you lose some recordings. Damn annoying but not the end of the world. Key point is you really can't minimise the chances with your buying choice. Chance of failure is 1%-2% each year. Flip a coin 7 times. If they all come down heads (or tails) then you have a drive failure.