I do NOT recommend repartitioning the external disk from Linux since you are using it for Windows 7 backups. It may not restart from Windows 7 afterwards if you are booting it with the system, or dump you into boot repair or just error out and force an MS fix on you when you plug it back in that will overwrite your Linux changes. Do the initial re-partitioning with Windows first. Once that is done you will probably be able to use Linux to set the new partition type so your LL can use part of it for backups. But by all means, if you must, forge onward as Steve describes and slam MS with the power of Linux until it dies like a lot of other unfortunate people actually do. You have noted that you wish to preserve your Windows 7 functionality on the drive. Defragment and partiton the disk from Windows. Make the new partion free space or fat32. Shut down Windows. Boot Linux and make the partition ext4 if you like, though that is not neccessary either for Linux backups.
TC