Perhaps someone can explain this to me...

My iMac contains a 1TB Western Digital hard drive (WDC WD1001FALS-403AA0), which is a SATA II / 3.0 Gb/S, or 375 MB/s, drive. With the BlackMagic Speed Test it reports a RW speed of about 100 MB/s, which is the same as I see reported from other sources. So, nothing wrong here.

Now I understand that the 375 MB/s is a theoretical maximum value of the SATA II bus, and that mechanical drives are not able to keep up with it. They do not 'saturate' the bus.

Just out of curiosity, what is the maximum speed of modern mechanical drives these days, regardless of wether they are SATA II or III?

BlackMagic tests of the new 2012 iMac also report about 100 MB/s, while the new laCie D2 with Thunderbolt port gives 180 MB/s. Assuming that the 'real' speed limits of the iMac drive is about the same as the one laCie is using..... why the difference? It would make sense to me that Apple would try to get the maximum out of the internal drive.. so why no increase of speed here?

I am considering getting a D2 TB-drive, but before I buy, I would like to get some more understanding of why the real speeds of one drive/bus combo is faster then others...