OneDrive and desktop storage issues

[Windows Central Question]

I thought that with Onedrive I would be able to store all of my photos that I currently have stored on a 'Seagate' (portable hard drive)? I have tried to copy them from Seagate to desktop which I thought would then automatically sync to Onedrive. However it says I do not have enough storage on my desktop. I am confused - the reason I bought Onedrive was to store everything I needed so that it is not on my desktop, so why if it is deleted on my desktop won't it stay on Onedrive?! Help!