Total Commander

I was trying to move files from my Brave Browser download directory to a different directory, both on sd card, and when they arrived at the new location, they were just filled with zeros. Any ideas? Messed up permissions? But destroying the files is pretty extreme!

It sounds like a fake SD-card, they are unfortunately a real plague on Amazon and e-bay. For example, the vendor claims that it's a 500GB SD-card or bigger, but it only has 32GB flash or so. When you write more data, it either doesn't write anything at all or overwrites older files. When you initially write to it you will not notice because the file is still in the write cache. But you will notice when you try to make a copy, or when you eject and re-insert the card, because it discards the cache.

Here is a free tool to test your card:
https://www.softpedia.com/get/System/System-Miscellaneous/H2testw.shtml