Let me start with a comment for future readers because so often I bump into people who do what I'm about to describe. Drive space getting low, dump important things onto an external and call it a backup, if you are moving things to the external then it is no longer a backup, but primary storage. In that case (not necessarily yours because I can't be sure how to interpret "started putting").
If it is indeed a backup, meaning it is a duplication of things active on your hard-drive then you are somewhat safe because the only problem is if both drives go bad simultaneously. Which isn't impossible, especially if you live somewhere that has lightning (or thieves).
to be doubly safe, I still like multiple local backups, in which one is swapped out on occasion and put in another location. In the end it costs less and the speed of restoration is much better. Even if the other location is just in another room (yes I know this is susceptible to the whole house burning down) it is better than potentially losing everything.
My favorite is an inexpensive NAS hidden away, using an NTFS file system so if the box goes bad I can still pull the drive.