A better way to protect against cryptolocker is to set up a NAS, that will force versioning of files. With force, I mean that all Changes of files will be saved on the NAS, and the client has no way to affect this. Then you save all important data on this versioned NAS. If the cryptolocker encrypts your NAS, you simply tell the NAS to rollback the files until the date where they is not encrypted.
Its important that the client cannot in some way touch this versioning. Eg, the client access the NAS and write a file. If the NAS sees that a already existing file will be overwritted, instead it will direct the write to a "change file" - eg something like a COW overlay but file-based instead of block-based.
When you notice "omg my files on the shared drive has been encrypted", you simply log in physically to the NAS and tell it to rollback the encrypted files to its previous version. If the files instead was deleted and a encrypted copy was saved, you tell the NAS to restore the deleted files.
Such a NAS can be built using a Linux server, samba and a couple of further software that allows it to keep track of Changes to files inside the shared drive.
The good thing with such a NAS, is that you kan safely keep the NAS connected and "mounted" all time. If cryptolocker touch it, you can simply roll back the time. Note that you need to build the NAS in such a way the client uses it as a simple shared drive. The client should NOT do the versioning of files itself, then cryptlocker can bypass it.
Its the NAS server that needs to enforce the versioning.
Yes, this means whole files will be transmitted to the NAS over the wire "unneccesarly", and then cut down to "difference files" at the server, but its really Worth it.
Using a external drive and just connecting it regularly for backup is NOT safe, cryptolocker can sit latent and wait for the backup drive to be connected, and then encrypt the files. You MUST use a NAS drive which enforces "versioning" on the server side.
This also protect against simply destructive payloads which simply deletes/infects files without encrypting them and without offering the possibile to pay a ransom to get the files back. (those are actually worser than the ransom viruses).