Makes a lot of sense in dual drive systems ... but still good for most of us ...

Most of the stuff on my C: drive is Windows, Microsoft Office, Drivers etc., things that get installed once and then over the course of a year there may be a few smaller updates. An SSD is perfect for that as it is fast access to stuff that will be read very often (every time I have to reboot for example, *grin*) ...

... having a separate D: drive (or whatever letter) for highly volatile data makes a fair bit of sense.

If you are the sort of person that downloads a couple of HD TV shows a day, then you may be using 2Gb of bandwidth a day (scale up as appropriate!) ... 40Tb would take you 20,000 days (about 55 years) to download ... so if you write 20Gb a day (which is a fair amount!) then that would about 5.5 years, 40Tb would be 1,000 days (so a little under three years). How long do you expect your spinning hard drive to last? I have some that have lasted ten years or more, and others that have failed in far shorter time ... MTBF/MTTF is a statistical average, hardware fails, hard disks fail *just before* backups would have been done ...

Or to put it another way, if you have a 10Mbps download, that's about 1MByte a second, 60Mb/minute, 3.6Gb/hour ... so you could save that data to your hard drive for 8 hours a day for nearly four years before you hit the 40Tb limit (and you'd fill your 60Gb drive in under three days)

(feel free to check my math, I may have got something horrendously wrong here!)

Assuming it has good load pattern analysis (and it should) and so uses the cells evenly ... for most purposes SSD is going to be good enough ... but not for, say, a daily backup drive that gets completely rewritten every day, as it will fail in 666 days! (40Tb/60Gb)

life span?

"There can be ... 120TB of data written to the 240GB Ultra"

That's only 500 full-volume write cycles. Which makes such a device very poor for large volume writes such as disk backups, and, given the way SSD handles erasure, only moderate for small volume dynamically updated data. So what''s it best for? Storing your photos and MP3s I guess.