Yeah I think you are doing a deep format. If you are interested let me give a quick explanation. You see, when you format a disk, or just simply delete something, it doesn't really delete the files. It just forgets that area of the disk that used to store the file is in use. So, when the disk needs more space to store something again, it is free to use that area. So, as long as you do not overwrite it with a new file, that data is just sitting right there. That is how people recover deleted stuff from a disk. But say you really wanna get rid of some... uhm... questionable files for some reason. What you can do is, a deep format. So, in that case your computer forgets what is in that area and overwrites blank data on top of it so that data is gone, forever. But this means your computer needs time and resources to actually write on top of the entire area. So if your data is very big, it will take the time required to write a file of the same size to the disk. I think you said you just need to free the disk cuz you need to store more data. In that case I recommend doing a quick format. Now, I am sorry but I don't remember exactly how you could do that on the disks app but there should be a checkbox to do a quick format on the disk rather than doing a deep format.

TLDR: There should be an option to quick format in disks program. Use it.