For a freelancer setup you only slow your work down by keeping everything on an external drive. In a design studio, where someone else may need to take over a project suddenly and the network is flaky, externals might make some sense. But only if the night crew backs them up!
The big plus of running from internal drives is speed. Use external drives for backup, where speed isn't such an issue, and for files you want to be able to cart around but don't want cluttering your laptop.
You must have a daily backup of all your work files and projects. It doesn't have to be on an external drive, but your safety backup must go to an external drive or a secure cloud service. Preferably, that drive should be kept in a separate location entirely.
Here's my freelance setup. A variation of this would probably work well for you:
My main workstation has 6TB of internal storage, of which a 2TB drive is dedicated to project and asset daily backup. A 1 TB drive mirrors the system drive. An external eSATA drive gets the weekly backup from the 2TB internal.
For working on the road, my main laptop is sync'd to the client projects superfolder so it automatically stays up to date as I work. I have a portable external drive with duplicate non-project-specific assets (photos, stock photography, stock audio, etc.) in case I need them. Any work done on the laptop is automatically sync'd to the workstation when it reconnects to the workstation over my wifi router, and is then backed up automatically. I can also access the workstation via LogMeIn if necessary.
For maximum production speed, having your system and program files (not swap files) on an SSD is close to optimum. Use spinning disks (we'll be calling them "old fashioned" within a few years) for data and images. But always work from internal drives for optimum workflow speed.