I do not know if an external USB 3.0 drive would be able to handle the data transfers of recording directly to it, but even if it could, I would not make that the primary choice, because the internal SATA 3 drive would be faster.
I definitely would store completed projects on the external USB 3.0 drive, however.
On my system - a desktop, I have two 2 TB non-SSD SATA 3 hard drives. I keep my OS and all applications on the primary drive, and store all samples and such - as well as all projects and their audio on the 2nd drive. I have NEVER had any bottlenecks running with this configuration.
There are many others here in the forum who run several more drives than me, and I am not opposed to doing so, it just has not been an issue for me to need an additional internal drive to split off either projects or samples. They just live happily together on the 2nd drive for me. (I do have a third drive - it is an external USB 3.0 drive - 2 TB - that I use for backups).
Since you already have your samples on the external drive, why not as a test copy them all to your new internal drive, and try running with projects and samples together on that drive? If you are not happy with performance, all you would need to do is to delete the samples from that drive and go back to the ones you already have on the external one. Just a thought.
Good luck,
Bob Bone