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I'm trying to resolve a major issue and hope that I can do so. A family member grabbed the wrong USB flash drive and they downloaded Windows 10 ISO to it. It's a Sandisk 15GB flash drive and there was tons of room on it, but it seems like it just overwrote the data that was on it. The extremely important things on there were text docs from Windows notepad, and pictures. I'm doing my best to try to follow the TestDisk PDF, but it's a bit overwhelming. I could really use some assistance to help if possible.

20.10.2025 modifed by recuperation, replaced "7" by "10"

According to a google search a Windows 10 ISO file has a size of approx. 3 GB.
Most likely all important metadata has been overwritten.
I don't see any chance using TestDisk to restore the overwritten partition (if you had just one).
Try out PhotoRec and check the resulting files if they are useful. Typically there are no folder and file names, though.
Before recovering anything useful, your recovery location will be filled with content of your ISO file.

For better results, try out commercial software.
PhotoRec is the last line of defense.

The Flash Drive is 15GB. The important data probably took up less than 200MB, the text files were barely anything at all. What was added to it was only 500MB, so there was plenty of room on it.

I tried using 3 different recovery software options and they did recover old deleted things that I didn't care about. The important things that I'm looking to get back it I didn't find.

I have ignored an imported fact in your first posting:

You wrote that the operating system was downloaded (not installed) on the USB flash drive. If it was downloaded as a file into an existing file system there is no reason for the other content of the stick to disappear.

On the other hand, people often claim the the software "Rufus" killed their USB flash device while (over-!)writing an image file onto it. The culprit is not Rufus, though, but rather devices with bad quality.

but it seems like it just overwrote the data that was on it.

This description lacks details about what existed afterwards.

Furthermore it is not clear if your "family member" executed any installer software or only downloaded an ISO file.

I tried using 3 different recovery software options and they did recover old deleted things that I didn't care about. The important things that I'm looking to get back it I didn't find.

Are you talking about TestDisk? PhotoRec? or three other commercial software products?

3 different recovery software options

is incomprehensible for me.

Case descriptions should be written in a way that leaves no room for interpretation.

I believe I figured out what happened. The wrong selection occurred and it turned into a bootable drive. From my understanding the data isn't recoverable when this occurs.

Thanks for your time, you can close out the thread.