In trying to delete files on my WD back-up drive, I deleted the last back-up (permanently), but that only freed up a couple of hundred GB. I.e. the drive still shows over 600 gb of used space (about 200 of this is a photo back-up, the rest is system files and other). I have downloaded the EaseUS ghost program and I am inclined to reformat WD back-up drive and then endeavor to use ghost to do a system back-up. Make sense?
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Since you are not 100% sure what other files may be living on the drive, I would choose caution over economics meaning that it's a far safer bet to just buy a new $60 external drive to ensure peace of mind that you have not lost any data. However, if you choose to use the current drive instead…

You just want to make sure you have all the photos on another drive as well and you are willing to say goodbye to any remaining files on that device. As we discussed, a common misconception related to backup drives is that people think they can store unique information on the external drive without it also being somewhere else as well. That is not the definition of a backup drive. If you keep your photos on an external drive as your main library, then those photos should also be on at least one more drive in addition to that drive. Perhaps not answering your question but I always err on the side of caution when it comes to photographs. System files are replaceable. Photos are not. In short, if you are completely sure you have copies of all the photos off of the WD external drive, and those pictures are also on yet another drive, then wipe away and use the old WD drive as your clone backup.