1000 years? Hype. That's a very large extrapolation. And irrelevant, anyway, even if true.

Technology changes. Things become obsolete. Storage media are no exception. How long were CDs state-of-the-art? Probably no more than 5% of that 1000 years will pass before the M-Disk is obsolete. Something better will come along.

For one thing, they're too small. 100 GBytes? Not enough. I have 1.2 TBytes of image data. After about 10 years of digital photography. I'm taking more photos than I was at the beginning of my digital photography period, and the images are getting larger so the files are also. I expect that in another 10 years I'll need more than 5 TBytes storage. Are you getting into video? Large files on steroids. You'll probably want 10-100 TBytes storage before too long.

If you're going to back up your files (and not just the image files, but everything else on your computer also) you shouldn't depend on any one technology. Change is the only thing that's permanent. At this point you have to distinguish between backup and archiving. Archiving is saving stuff for posterity that you probably won't be using much (but someone else will). Backup is for current stuff that you might want to use again within a few years.

Archiving requires more than backup. It requires some rosetta stone to be included so that the future data users can identify what's in the archive. And how to find it, and how to interpret it. It's not enough to have an image file. The image has to have associated keywords, ID of people and locations in the image, some explanation of what's going on. You've got all that stuff in your head, but you have to make your head available to the people that will follow you.

At any rate, both backup and archiving require maintenance. When technology changes you have to transcribe all your stuff onto the new technology (maybe after a couple years so they can work the bugs out). Otherwise you will have a closet full of wire recordings with no player.

With technology improving by the minute, there are many ways to store your images. Since external hard drives are so inexpensive, this is probably the #1 way of storing them. I store mine on a 4TB external drive and back them up on another 4TB external HD. I used to use 1TB external drives, but then the USB3 externals came down in price to I upgraded to the USB 3, 4TB drives about 2 years ago. I also found that using solid state drives inside the computer breathed new life into it as well so I installed 3 of them into my computer.

Solid state external drives are starting to come down in price and USB 3.1 which is double the speed of USB 3.0 is out now. So I suppose when they become larger and more affordable I will upgrade to those eventually.

I've never been a fan of clouds, because they are extremely slow hampered by the limits of your internet speeds. When you are talking about 3TB of data that needs storing, it would take 3 weeks to upload and/or 3 weeks to download from the cloud. Seriously!

If your computer has sufficient storage space, there is no harm in having them there, as well as backed up to an xHD. What you are missing are added backups. You may believe that if your computer failed, you would be safe with the xHD. And maybe you would be right. BUT all HDs can fail, for multiple reasons, and at least one more additional back up drive is a good idea.

Professional photographers are right to be fanatic about ensuring the safety of their images. This is why off-site storage and/or cloud storage are so often recommended. This works for those of us who are not pros, but only if we want to go to the trouble and expense. Ask yourself how you would feel if all copies of your images disappeared at once. That ought to tell you whether you ought to go for some additional storage away from your home/studio/office... The nice thing about xHDs is they can be moved to another site, and buying them is a one-time expense. I have not tried Cloud storage, but my understanding of it is you can access your images from any computer, so if something happens to your other backups and computers you can use another computer and retrieve them all.

Susan, You can get a WD "Personal" Cloud which you own and maintain in your home or business for about $160 for a 4tb drive. The 2 TB drive is about $130 or so. They also have a usb connect so that you can add a 3tb external usb drive to the cloud. I have a 4tb personal cloud and this. https://www.bhphotovideo.com/bnh/controller/home?O=&sku=1244355&gclid=Cj0KEQiAzsvEBRDEluzk96e4rqABEiQAezEOoFzomUYosHHnr9U-ew8EU70KSJqqgX3cS7e_jP_9FI8aAs578P8HAQ&Q=&ap=y&m=Y&c3api=1876%2C92051678882%2C&is=REG&A=details
In which I have 4 drives 4tb each. They are both accessible from the internet and all sit inside my firewall so that they are protected from unauthorized access. I use the 16tb for my "cloud" backup and it really could be located anywhere that had internet access. If my office wasn't in my home, then I could that the cloud at home and back my office computers up to it. Just food for though. I also that the 4tb personal cloud and I got it from Sam's Club. I think I paid about $140 or so for it 3 years ago and it still works fine. In fact, I liked it so much that I bought the Chassis Personal cloud and the drives to populate it. By the way, since it holds 4 drives, you can buy the drives one at a time and add as necessary.