[Sirtel]

Copying the library folder to Dropbox works pretty well (don't put your original library folder in there, only a copy). For subsequent updates, install FreeFileSync and use its mirror option to copy any later changes in the original library to the backup library. This way you won't need to copy the entire library over every time you make changes. And always make changes in the original library, not the Dropbox one - only mirror the changes from one to the other, not vice versa.

Don't use Google Drive for backup, it will destroy your library.