Which would be better to install Megascenery photoscenery for use with P3Dv4? I bought the entire US on sale with the drive, and the bits were delivered on the drive. My preference is to just add to the scenery library and run from the USB drive as is. Am I better off installing to an internal SATA drive instead? 

No one with any opinion on using a USB 3.0 external drive as a scenery location for photoscenery?

SATA 3 SSDs have a higher transfer rate than USB 3 drives so transferring large scenery files would be quicker from an internal drive. An internal drive would also avoid the potential problem of USB drive letters changing for no apparent reason.

Since you said you have the whole US on there, I would pull the case apart from the portable drive and install the drive into your computer if you have an extra bay.... Or just purchase and install a new drive and image the USB3 drive to the new internal drive. (It will be MUCH faster then copy and paste). IF you cant get a SSD big enough get the WD Black drive they spin faster and will help with load times.