HDD's, even hotter running scsi ones give a small amount of heat relative to their size so you really don't need to worry about the copper quality, The heat is more heat soak over time, so as long as there is a reasonable contact area between block and HDD at some point, and the block has some flow of coolant through it the drive will be keep from getting to hot. heat will always migrate to a cooler surface assuming a good thermal conductivity.
Block design depends on other factors of your cooling set-up, this is mine and keeps two Seagate IV 7200 IDE's at around 20c with 14c coolant
