Hi Eva, Photoshop uses part of your hard drive as a temporary store . . .
When you make changes to a photo, it stores each change made to the photo as a separate image file in the scratch disc, so you can go back through the history and revert to that version of the photo
If you have another drive on your PC or a USB flash drive, set Photoshop to use that as a scratch disc, Photoshop does not like using the Windows drive as a scratch disc
In Photoshop, go to Edit - Preferences - Scratch Discs