I got caught out by exactly this, and I'm not exactly tech illiterate. what made it even more annoying is by the time I'd realised what had happened, it was practically impossible to get the files back out of OneDrive (since I decided that this was enough Windows for me, and went back to Linux), since the webui does NOT handle downloading lots of small files well, and you just end up getting a partially complete zip file. I gave up in the end as nothing in there was particularly important. This is an incredibly annoying default.

I had same exact experience with macOS and iCloud. macOS by default enables offloading Documents to cloud, transparently. Problem is if you try to get those files back to store them offline, it gets very tricky very quickly with ambiguous verbiage and lengthy process that you never actually know status of. I ended up losing some files as a result, which came as a total shock to me. I was already in the process of moving back to Linux (hence downloading of the Documents) and this was final straw.

From their point of view, that's the product working correctly. The whole point of all these consumer cloud storage products is to make it easy to upload your stuff and impossible to download it. (impossible - 1u to be precise, for legal purposes).

iPhoto does this the best. Its default is to upload every one of your photos to its cloud and delete the original from your phone. Then if you want it back, you can just click on the one photo you want and like magic, it's back on your phone. Want it on you PC? No problem. Open the web interface, click the one photo you want, and there's even a download button.

Want all your photos? Oh. Well, you can just click each one of them then click the download button.

I mean, sure, there's also this icloud app that will slowly download your entire photo collection into a single folder on your computer, slowing down the entire time before eventually grinding to a halt by the time it has put 10000 of your 250,000 photos into that folder. Of course, you can restart it, but it'll start again at the beginning.

But yeah, that's the business model. Put your stuff on the cloud, make it hard to get it back, charge you to keep it there.

Oh and another fun thing! I eventually just emptied the OneDrive so Microsoft would stop bothering me. This was maybe six months ago or so. Microsoft confirms I am storing nothing there. Just a week or two ago I got yet another email begging for money because my OneDrive was apparently full. It was a genuine email, I went as far as checking the headers for SPF/DKIM. When I signed into onedrive, still empty!

I wouldn't use the webui for that. Getting rid of onedrive in favor for a self-hosted nextcloud, I used the native client to download all the files on the machine and then moved them out. This also removed them from onedrive after acknowledging the "A lot of files have been deleted from your onedrive account" warning. Actually deleting the onedrive application was also not as straight-forward as some other users may want you to believe. Even now, I'm not sure it won't just pop-up one day once again.

rclone supposedly supports transferring files from and to OneDrive.

I wonder if rclone would have behaved any better than the web UI.

I too have seen onedrive do this to people who aren't super-heavy computer users. Onedrive is a menace.