The speed differences from what sounds like USB 2.x versus SATA. As to the copy, you can only know if you had created CRC checksums for the files and tested before and after. However folk will copy a billion files and maybe one error using either method. In other words, should be fine.
I am trying to move a 500+ GB database from one computer to a newer computer that has an SSD drive. Transferring from the original to a USB external drive then transferring that to the looks like it will take 15 hours. I cancelled the transfer, opened the original computer removed the hard drive and connected it to an empty Sata cable in the new computer. I pulled the power cable from the CD drive and plugged that to the hard drive. I was then able to navigate to the old drive and am now copying the database to the desktop. Looks like it will only take 2 hours. Is this OK to do? Is the data being properly copied? Any issues with doing this? (If not, why don't they use sata cables for external hard drives?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic