IO numbers that everyone should know
The bottom line is that unless you're thrashing the HDD with lots of 4KB random writes, the HDD should not be tapped out till about 30+ MB/s (and an SSD should be just fine till about 150 - 300 MB/s). If you're seeing an HDD tapped out at 3 MB/s, then you're either not writing sequentially or your block size of writes is too small. If you're seeing an SSD tapped out at < 100 MB/s, it's almost certainly a software bug and not an IO limitation. In either case, the basic norm holds - if you can, always use SSDs, they usually save you money in CPU time.