What's the blackmagic speed test score for 2011 iMac 27" be?

That is about the right speed for an internal HDD, though they can achieve up to 120 MB/s.
Have you tried AJA System Test or Xbench* yet to benchmark your HDD?

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Anyway, the SSD in the MBA is at least twice as fast, that is why SSDs are so desired.
I have an SSD in my 2007 iMac and it feels like a new Mac, though the CPU is still "slow".
I also have an SSD in my 2009 MBP and it is also very fast. HDDs as boot drives are very slow compared to an SSD.

 

I just switched out my HDD to an SSD yesterday and ran XBench and BlackMagic right before the change and the results varied from 120-145MB/s.

After the switch to the Corsair Force GT 120GB I jumped to 485MB/s as of 10 mins ago when I checked.

 

The Mac does not matter. The HD speed is limited to the ability of the manufacturers HD product Apple put inside the box. The Caviar Black in the Mac Pro's can only peg 85-95MB/s but it's got great response time. Bandwidth is not everything. I also have a WD 2TB Green than can get upwards of 120MB/s on those tests but is slower overall compared to the Black. The fastest mechanical HDD can get 140-145MB/s, that's it. An SSD can get 560MB/s+ with the right combination of tech. A macbook air has a faster HD than all mechanical HD shipping Mac's. But all you have to do is buy an SSD and you too can have "instant on".