Backing Up External Hard Drive

I use my laptop to DJ.  I have an 256GB internal drive, however my library of audio files is over 1TB.  I currently have my audio library stored on an external hard drive and have a 2nd one as a backup.  I would like to copy my entire library to a folder on Dropbox and use Smart Sync to store them online only - copying files to my local drive when I want to "spin" them.  I constantly add more files to the library, but would only back up to Drop Box monthly.

I have read some other articles and I understand that DropBox is not intended to be a backup solution.  In my daily working on my radio show and live sets, I find it cumbersome to constantly be hooking up my hard drive to run through songs and think accessing it through DropBox would be a good solution (In the past, I could fit my entire library on a second internal hard drive - but as my library has grown, and my new computer's internal storage has shrunk, I am unable to work like this now)

Thanks in advance

G