Hey there, Smj___.
What do you mean by "it's too full to open"? Check if the HDD is recognized in Disk Management and please post a screenshot of what you see there (you can use imgur.com tp upload the image)? Here's how to open it:
How to access Disk Management in Windows to manage hard drives.
You could try the drive with a different USB cable and a different computer, to see if the same thing happens. Other than that you could try defragmenting the HDD as well (it a Windows feature although I can't post a tutorial since I don't know which is your version of Windows). Other than that running "chkdsk /r" (open CMD as administrator and type
chkdsk X: /r, where X is the letter of the drive you wish to check so don't forget to change it with the appropriate one) might also do the trick. However, note that this should be done as a last resort, since it's possible for it to make things worse instead of fixing the issue if the data has been corrupted or the partition.
Please let me know how everything goes.
Boogieman_WD