New WD Elements External Hard Drive, but can't drag files on it?

Hi everyone,

I've recently got myself a new hard drive as mentioned before. But, when I had plugged it on my MacBook Air using Version 10.12.3 and OS Sierra, I can't seem to drag any type of files onto it. I don't understand what type of issue that's causing this to happen. So when I tried to drag a file onto it, it shows a block symbol. Would my pop-up warning of 'Your disk is almost full - save space by optimising storage' have anything to do with this? With only 4GB free storage available, would this be an issue? I remember, from my previous 1TB WD Elements hard drive, that there was a backup software included in it, and this 2TB doesn't seem to have it. Do some hard drives don't include the backup software? I don't want to do anything risky with my hard drive until there's a safe option of knowing how to diagnose the issue. I appreciate any solutions that might help me, if you had the same problem before and resolved it.

Thanks heaps!