The position of the external case, standing up or laid flat, could impact the drive's performance...
Awhile ego, I've built a system with two identical 1 TB Seagate SATA 3 drives; one for internal and the other for the external case picked up at Newegg. When the performance of the drives were tested, it was interesting.
Internal drive performance:
eSATA drive performance, upright:
That's close to 20% performance drop for the eSATA with the same drive and it didn't make much sense since the eSATA on the machine in question is actually SATA 3. Just out of curiosity, I've laid the external drive flat and ran the test again; this is the result:
That's better and couple of percentage point of differences could be related to the CDM in itself, to the fact that the external drive had couple of gig data on it, eSATA cable, etc.
The HD docks, with their vertical placement of the drives, may have similar results. I don't have one to test, but would be nice if someone could test it...