When you view the "Disk Activity" section in Activity Monitor, you can see the read and write speeds of the system's hard disks both in current values and in a small history graph. Some people have wondered if current SATA II hard drives included in modern Macs have 300 MB/sec transfer rates, then why when using them the drives only transfer at around 25-35MB/sec, even when copying large files.
The 300MB/sec transfer rate for SATA II drives is in actuality the maximum speed of the SATA bus, and not the true speed of the drives. This theoretical maximum can technically be achieved if you cache information in the bus and under optimal conditions send it through the bus to another controller. In reality, there are numerous bottlenecks, and most mechanical hard drives have a performance maximum of about 80MB/sec, though some high-performance ones can peak at between 140 and 160MB/sec. However, keep in mind that these numbers are usually determined under optimal conditions. In real-world use, performance of these drives will likely be significantly less.
Some of the factors that will affect the maximum performance of a drive are the following:
Buffer sizes--Drives will read and write at different rates for different buffer sizes. In general, smaller contiguous chunks of data will take longer to process and send through the bus than larger contiguous chunks, but these properties will depend on the drive's cache sizes, and manufacturing and firmware technologies.
Seek times and fragmentation--Even though you may be copying a large file, the data from that file may not all be contiguous on the drive, and may be fragmented into several or many chunks. Therefore, in addition to the potential for different fragment sizes to be handled at different rates, the drive may also have to spend time seeking each piece of the fragmented file. There may also be numerous rotational delays introduced when reading or writing a file, where if the drive head reaches the target track but misses the target block, it will have to wait for the drive to do a full revolution before being able to read or write. The more fragments a file has, the more that rotational delays can be a factor.
Not only might the source file be fragmented, but the free space to which the drive is writing the file might also be fragmented and require the drive to perform additional seek and write actions when copying the file.
Source and destination on the same drive?--While copying from one drive to another can be impacted by fragmentation and buffer size handling, the inherent delays may only be augmented by having them both be happening on the same device for the same copy routine. A drive that copies a file from one location to another will have to seek potentially fragmented source file chunks of different sizes and then write them to potentially fragmented free space on another section of the drive, causing the drive heads to move all over the place.
Other system processes--The last factor that can affect performance is when other system processes use the drive, and how the system uses the drive. For instance, when a file is written to there may be a number of operations being performed on it, including initial access, reading permissions properties from the directory, journaling changes, and then writing those changes to disk. In addition, other features like virtual memory usage will require hard-drive time, and can impact read and write performances.
Overall while disks are packaged with bus connections that can support 300MB/sec or greater transfer rates, an average consumer hard drive that gets 80MB/sec will likely only do so under optimum conditions. Depending on what is being transferred and various potential interferences, it is not uncommon to have real-world transfer rates be closer to 30-40MB/sec.
Lower than optimum performance does not mean your drive is failing or malfunctioning; however, if you do start seeing either excessive drive use or instances where the drive's transfer rates fall sharply and stay low, then you might consider using a robust drive utility to test the drive. Apple offers Tech Tool Pro for AppleCare subscribers, but other options include DiskWarrior, Drive Genius, and DiskTools Pro. You can also install a utility that monitors the S.M.A.R.T. (internal diasgnostics) status of your drive, which is one way to see if a drive is experiencing any problems that can be detected by the S.M.A.R.T. system. A couple of these include SMARTReporter and SMART Utility.