There's many different reasons why it's not a a good idea, one reason it is because your internal drive is already under the heavy pressure of running Final Cut or After Affects, and by having to deal with your footage it's just too much, if you do a little bit or research you can find out exactly why. When I started editing I started with a 2008 MacBook Pro laptop and I first started with Final Cut Pro 7 then learned to use X, I worked with Full HD footage transcoded to PRORES 422, I invested on a 1TB external drive with a firewire connections because the tutorials that I watched recommended that. All my editing ran smoothly and my computer would only take a long time during transcoding and rendering but this was because of my CPU and no because of my drive. You probably get a lot of messages of frames being dropped and what not, this also has to do with your internal hard drive not being to keep with your editing software, remember all your footage should be on an external hard drive, and your editing software stays on your internal drive. Based on your specs you shouldn't have any problems editing with final cut, if I were you I would increase the ram to 8GB.