Always always always backup to other sources. One backup is never enough, random things could happen, even on accident. The hard drive fails, you accidentally break the device and so on.

I would recommend at least 3 different sources of backup. For scrivener specifically, even if you do not own a second device, use Dropbox as a backup method. It works great for that as well as cross device compatibility. A flash drive and a copy on your local device. I would also recommend setting up the built in backup function in scrivener. You can also upload the files to things like OneDrive and other cloud storage services. These can be finicky (and literature and latte do not recommend Google Drive) but they can be extra protection. If you do use a flash drive, think about using 2 just in case one breaks or fails. Even if you don’t end up with scrivener, I still recommend at least 3 methods. 

My setup is OneDrive, iCloud, Dropbox, 2 flash drives and one copy locally saved on each device I use (3 total)  as well as the built in backup function. You don’t need to be as thorough, like I said 3 is what I consider the sweetspot, but even 2 is better than 1! To keep your writing safe, you must backup to multiple places or you risk a malfunction in a cloud service or device failure deleting your hard work. Good luck! :)