The read and write speeds of the drives are different which results in a variance when transferring files. The badwidth between the drives might be 300mbytes/sec but the drives themselves maybe unable to read or write the data fast enough to take full advantage of the bandwidth. Also if one of the drives has other duties, swap file, system disk etc. That can eat into it's read/write time as well.
What you are getting is normal. Well, except that I THINK you are confusing the b's.
Friends who don't understand may have been deluded by the specs without reading them fully. SATA original spec has a MAXIMUM data transfer rate of 1.5 Gb/s, and SATA II is at 3.0 Gb/s That is SMALL b, meaning "bit". Most data transfers are measured in BYTES, and the symbol for that is CAPITAL B. As a ROUGH conversion, 1 Byte is 9 to 10 bits. Actually, a Byte is 8 bits in most systems. BUT some data storage and retrieval systems actually use a ninth bit for error checking, and any data transfer process has a small amount of overhead, so I prefer to do the simple math and approximate one Byte with 10 bits.
So, my way of doing it is that original SATA could do 150 MB/s MAX, and SATA II could do 300 MB/s. BUT that is the MAXIMUM rate of data transfer along the communication channels between the HDD's buffer RAM and the mobo's CPU or RAM. To get the data from a spinning disk into the HDD's buffer RAM involves several mechanical movement systems - disk spin speed and head seek speed, and a few other electronic considerations - that cannot keep up with those Max speeds. So if you read through good hardware reviews (Tom's has many) you will see what actually can be achieved in an AVERAGE data transfer rate over a long file. In fact, the average rate depends somewhat on the types of files and on the average file size, so those factors need to be taken into account.
Bottom line seems to be that a good SATA II (3.0 Gb/s) HDD will often achieve a 70 to 100 MB/s long-term average rate, and your fast blacks are doing that. I don't know for sure what older IDE drives can do, but I know it is less. For an external HDD connected by USB2 (a slower interface than SATA), the rate is more like 30 to 35 MB/s (USB3 is faster, close to SATA II rates). Firewire 400 is a little slower than SATA II, and Firewire 800 (uncommon on PC's but on many Mac's) may be faster than SATA II. IF your external HDD is a SATA II and connected by a good eSATA port to your machine, it probably will exhibit a speed close to an internal SATA II drive.
Now, none of those speeds come close to the original SATA spec of 150 MB/s max, never mind the SATA II spec twice as good! I understand that some HDD's do get up to, and maybe a bit faster than, 150 MB/s. NO mechanical (i.e., spinning disks and moving heads on arms) hard drive can reach 300 MB/s now, and it's doubtful they ever will. So why SATA 6.0 Gb/s? You can get closer to those speeds if the data device is NOT mechanical - on the SSD's.
I found a posable solution to the slow transfer rate. I am not computer minded so this was a total fluke. I had slow transfer speeds on one of my wd 3tb hard drives of 20mbps. While the other was at 107mbps. I did the swapping of hard drive bays and now the one that was getting high transfer was now also at 20mbps. I was stumped i did lead changes and everything to both and when i had both drives not in computer box just sitting on top my speeds were above 100mbps. The issue was right in my face. A cooling fan was blowing over that hdd bay area. I did research and found that their are 3 breathing holes on the drive. Why or how air can effects hdd i dont know. Hope that helps.