I had backed up all my LR photo files in an external Seagate Backup drive. Now, if i delete the contents of all "My Lightroom Photos" folder in my Mac (these have been backed up in the external hard drive) thus rendering them blank how do I work on future post processing with LR5?
You can connect to the photos on the external HD via Adobe Lightroom - Find moved or missing files and folders
Can I export directly to the external hard drive?
Sure, you can set the export dialog box to export to any disk you want
If not, and if i continue to export them to my Mac's hard drive when this is backed up does it back up the newly created photo files while still retaining those in the Backup Drive?
Lightroom does not make backups of your photos, and exported photos are never in any way backups of your originals
Technical detail: these files you have moved to the external hard drive are not "backups" ,... do not think that you have created backups of your photos, which I consider mandatory ... these are not backups because you say you are going to delete the originals ... you need to have at least two copies of your photos (one copy on one disk and another copy on a different physical disk) to have backups, making real backups of all of your photos and catalog file is extremely important and in my opinion mandatory