I stand corrected. Cheers for that.

So what you saying is all SSD fast but data degrades over time. Best to back it up on a mechanical drive (or two lol)

SSD's most certainly "degrade" over time.
It's called "bit flipping".

Essentially what happens is that the 1's and 0's in a NAND cell "flip" due to electron leakage.
Increase the number of states in a cell and you increase the likelihood... I.E. SLC is 1-bit so it's the most reliable... Consoles are using 3-bit or 4-bit NAND which increases the likelihood of that happening.

The same thing can happen to mechanical hard drives as well, but it's far less likely and they can handle being "stored" far better over long periods of time.

The most reliable consumer form of storage is going to be multiple mechanical hard drives in Raid so you can get some redundancy happening, obviously the performance isn't at SSD levels... So that caveat is an issue.

Storage on Series X - 802GB available to use. Also 920GB on the expandable storage. I wonder what the Series S is going to be.

Well, we now have an answer to how long it will take to move:

One of them comes from GamesBeat's Jeff Grubb who tested how long it takes to transfer files between various drives. In the video embedded above, Grubb moves Assassin's Creed Origins (which is a 49GB file) around his storage locations in different configurations. Here's what the tests found.

Moving Origins from an external SSD to an internal SSD took only 2 minutes and 18 seconds. The inverse, going from internal SSD to external SSD, took 4 minutes and 33 seconds. Probably most important, transferring from an external HDD to an internal SSD took 7 minutes and 46 seconds. Finally, moving from internal SSD to external HDD took 10 minutes and 36 seconds.

That third result is the one most people will want to keep in mind. It's the one that dictates whether they can live with swapping installs back and forth or whether they'd rather pony up for the official external SSD. Approximately 8 minutes to get the game where it needs to go in order to be played. Assuming next-gen installs are much bigger -- let's just call a file 100GB for convenience's sake -- it'll take about 16 minutes to transfer a game.

https://www.destructoid.com/stories/xbox-series-x-has-speedy-file-transfers-between-hdd-and-ssd-605065.phtml

I think id rather pay $220 for the next gen storage expansion for Xbox.https://t.co/fcfcdhI0au

— Pete in 3D (@VidGamesPete) September 28, 2020

https://www.microsoft.com/en-us/p/seagate-game-drive-for-xbox-ssd/9082KXHLZ2XR/LCR8?activetab=pivot:overviewtab

Considering the external 1TB SSD is $199 that makes the $219 expendable storage seem reasonable.

Yeah the 7 minutes for AC at 40GB is roughly 100MB/s. I'd love to know what HDD he was using because in theory with something like a 4TB WD Black HDD you could do it another 40% faster. 

This is clearly the better way to go for me. On PC I have a 500GB NVme and then a stack of really average sata hard drives (I think 6.4TB totall) i've acquired over the years where I stick my strategy games, movies and pictures, and games I've lost interest in. I'm used to moving things over and I will continue to because flash storage is expensive everywhere.

Or a MyBook Duo (Raid with 200 - 400 MB/s).

I just tested the copy time of Assassin's Creed: Origins (PC-Version, 71 GB) from my 28TB-HDD to my SATA-SSD: 03:36 minutes for the last 63 GB, ~4 minutes in total:

2:18 min (138 seconds) to move or copy 49 GB ain't that impressive for NVME-SSDs, that's just 355 MB/s.

An external HDD-Raid ain't that much slower:

A Western Digital Mybook Duo can do 364.63MB/s read/353.52MB/s write in sequential... 20 Terabytes, Raid 0. Hard to pass up in my opinion if you want cold storage for the entire generation with additional redundancy.

This will be my approach I think.