SpiderOak keeps your files secure while in online storage and while in transit between your computers and our servers, but is not intended to secure the copies that remain on your computers. The way to protect the files on your computer is to (1) use full partition or disk encryption and (2) log out of your computer before leaving it unattended or loaning it to others. Local encryption is most secure when done at the filesystem level through the operating system, which is why we at SpiderOak don't try to bolt it on after the fact. All modern operating systems have provisions for full disk or full partition encryption, and we strongly recommend their use:
Windows: BitLocker http://windows.microsoft.com/en-us/windows7/products/features/bitlocker
Mac: FileVault http://support.apple.com/kb/ht4790
Linux: LUKS, built in to most major distributions. For example, on Ubuntu: https://www.eff.org/deeplinks/2012/11/privacy-ubuntu-1210-full-disk-encryption
SpiderOak and local encryption solutions such as TrueCrypt do different things. SpiderOak encrypts your files when stored on the cloud and in transit between your and our computers, but not while on your computers. There are, however, some third-party solutions that let you create an encrypted disk image which SpiderOak can then back up.
On Mac, use the Disk Utility - http://mac.appstorm.net/how-to/os-x/creating-disk-images-with-disk-utility/
On Windows, Mac, or Linux, one option is VeraCrypt, a TrueCrypt replacement - https://veracrypt.codeplex.com/
To use encrypted disk images together with SpiderOak, first launch the disk utility and select the folders or files to be included in the disk image, then follow the application's directions to set a password. After the disk image has been created, simply add it to your SpiderOak backup set in the Backup tab of the SpiderOak application. The next time you need to access the files in the disk image, use the encryption program to decrypt the disk image, edit your files, and then reform the disk image in the same location as the original disk image. SpiderOak will de-duplicate the disk image to save space in your account.