The other HDD won't sustain the higher rate because it is USB2.0
What? this is a USB 3.0 connection. My issue is that when writing from an internal SSD to an external SSD connected by a USB 3.0 wire the speed is limited to ~50MB/s but when I write files from the external SSD to the internal SSD I get ~150MB/s. How can the connection one way be 3x faster than the other when both drives are SSDs and not close to their max read / write speeds? I only included the HDD speeds to show that the cable itself can at least sustain 100MB/s both ways in case it was some funky issue.