When configuring deja-dup through the "Backup" dialog in the settings you can select a disk in the "Storage" section. When doing so you only get an empty "Folder" input field. This happens for local as well as external disks. Not often do you want the backup to be placed directly in the root of the disk though.
It would be more efficient to provide a "Choose Folder..." button, just like when selecting the "Local folder" option from the "Backup location" dropdown. Only in this case it would use the selected disk as a base. This would allow you to quickly choose a subfolder and/or create one.
ProblemType: Bug
DistroRelease: Ubuntu 11.10
Package: deja-dup 20.0-0ubuntu3
ProcVersionSignature: Ubuntu 3.0.0-12.20-generic-pae 3.0.4
Uname: Linux 3.0.0-12-generic-pae i686
NonfreeKernelModules: fglrx
ApportVersion: 1.23-0ubuntu3
Architecture: i386
Date: Thu Oct 20 14:07:12 2011
EcryptfsInUse: Yes
InstallationMedia: Ubuntu 11.10 "Oneiric Ocelot" - Release i386 (20111012)
SourcePackage: deja-dup
UpgradeStatus: No upgrade log present (probably fresh install)
There is a "Folder" entry box that lets you enter a subfolder of the storage disk. You're asking for a button that lets you fill that out via a file dialog?