If I understand this correctly, the three scans and the TRT are of exactly the same disc (burned in another drive), and the two first scans are in exactly the same 1693S drive but mounted externally and internally?
A drive can show much worse scans when it heats up after performing multiple burns/scans/reads. Could the first scan above in the externally mounted 1693S be the result of a drive being really hot after doing a lot of work, while the second scan is in a cool drive?
I suppose an inadequate power supply for the externally mounted drive could also contribute, although that is merely a hypothesis and not based on any experience with insufficient power supplies.
I suppose it's also possible that you can get scanning differences due to excessive vibrations in the disc and drive, and vibrations could be significantly different depending on how and where the drive is mounted. Since the scans are done at only 4x this doesn't seem very likely.
This is mostly conjecture I'm afraid, but the part about scans becoming worse when the drive is overheating is real enough, but it might not be the right explanation or only part of the explanation.