Speed is dependent on a multitude of factors. I will list appropriate ones here. I'm guessing max speed should be around 70-90MB/s for 5400rpm WD portable drive.
- Your Macbook HDD Write Speed is slow 30/35MB <-- limiting WD 2TB Portable 5400rpm file copy
- The data you copy is a multitude of over ~500 files thus slowing down copying
- Data is Fragmented, files are split physically on the portable HDD, thus requiring time to seek and return.
- Other programs are using the Portable Disk, such as virus scan
- You enabled drive encryption, this slows things down based off encryption speed
- "Chance" that your HDD is failing, use HD tune to www.hdtune.com check that