It is possible to backup and sync with hidden volumes that do not have a drive letter and are not mounted. To see which volumes are or are not mounted, and discover their unique Volume GUID, go to the command prompt and use the command mountvol

You will then see a list of all the volumes on the system and where they are mounted. The volume GUID's have the format \\?\Volume\{GUID}\

Use the volume GUID in the source/left or destination/right path in your profiles configuration like any other path, for example \\?\Volume{1962a3df-5101-11df-ae1d-806e6f6e6963}\My folder\Sub folder\

Alternatively, with SyncBack V11, you can set your source or destination path to \\?\ and then click on the folder icon in the source/destination edit box to navigate to the volume you want to use: