SafeCrypt works to encrypt Dropbox, Google Drive, OneDrive, Box, Bitcasa, and any other cloud storage provider that allows you to create a folder.
You can also use SafeCrypt to encrypt local files, network drives, and external media such as CDs, DVDs, USB flash drives and hard drives.
For example if you want to encrypt files on a USB flash drive, simply use the local folder option and store the SafeCrypt files and folder on your flash drive.
If you choose to use SafeCrypt software to encrypt external media, each person using the media (CD, DVD, flash drive, hard drive) would need to have SafeCrypt installed on their computer to decrypt the data.
Each SafeCrypt software license will work on up to 3 computers and the iOS and Android app are always free to use.
SafeCrypt can also be used with other cloud storage accounts that are not listed in the options simply by selecting to use "Local Folder" when creating a SafeCrypt encrypted drive; to do that you simply navigate to the shared folder for the cloud account. (for example I tested using SafeCrypt with JottaCloud, and it works perfectly)
Be certain to always create a new folder for SafeCrypt in whatever directory you choose, or it could negatively affect other files or programs located in that directory.