Manual Run SCCM 2012 Backup

This is the case, You can set the SCCM 2012 to schedule a backup and set the destination backup location.

This is good, but what if you want to run a manual backup, its easier than what you may expect and also its in a place that you not expect (not in the Configuration Manager Console), follow this path

Open Services.msc and then search for the following service SMS_SITE_BACKUP and just simple start it.

At this point the SMS site backup should start, refer to the smsbkup.log file (located in D:\Program Files (x86)\Microsoft Configuration Manager\Logs) 

Hope this that help