I am running Windows 8.1 and downloaded Windows 10 for installation on another machine. I have a 128 B SanDisk Cruzer Blade USB drive that had other backed-up files on it.
After the files had been loaded by the Media Creation Tool all of my other data on the USB drive was gone and now it is only the 3.13 GB of data for Windows 10 and 28.8 GB free space. Where did my data and the other 90GB go?
The media creation tool formats your USB drive, which wipes all of the data on it. It states when you download the tool:
A blank USB or DVD (and DVD burner) with at least 4 GB of space if you want to create media. We recommend using a blank USB or blank DVD, because any content on it will be deleted.
http://www.microsoft.com/en-us/software-download/windows10
Screenshot from the tool itself (after selecting to create a USB installer):
