As athomson says, copying via FTP will not decrypt anything. Whereas the Hummy’s own copy facility. (To USB) will decrypt SD recordings.
If you do not use the Custom firmware (which happens to be a superb package, by the way) then there is a PC based tool called Foxy that you can use to enable the Hummy copy to decrypt HD recordings on a file by file basis. It does involve copying a file (.htm file) for each recording to the PC, running Foxy and copying the file back.
Best solution is to install the Custom Firmware. It can then automatically decrypt some/all recordings automatically. These can then be transferred to the PC via any copy software (Try FileZilla) ready for you to use as you wish.
Admin edit: Links and follow up removed.