What am I doing wrong? Transferring files/ freeing up space

I am trying to backup my Procreate files. I have plugged in the WD Passport wireless Pro with an Apple adapter. I see the drive in menu of my Files app. I open up Procreate along side the Files app and drag over files. It worked the first time, but now my iPad says I’m running out of space and notHing else will transfer over. Why is the iPad (Pro 11 inches) acting as if I’m adding files onto my iPad? I went to storage management and I believe that my available space is lower than before, though I didn’t check to see what it was at before doing this. I’ve tried with the drive plugged in, as well as doing it wirelessly via My iCloud OS 3 app. I get the same response. This doesn’t make any sense to me! Help!

Rebecca