I have a 6B300S0 with a load of data on it that I am desperately trying to get off it. The transfer rate has dropped to almost nothing and the system become really unstable. HD tune returns these errors:
Reallocated Sector Count - failed, the scan revealed only one 114MB sector damaged...
Interesting that you get some throughput from a damaged drive. I would go with the second poster's recommendation. Are you copying files off it from within the original machine (or is it the C: on the original machine)? if that is the case I would suggest taking it out of the computer and copying the files on a different system. Maybe there's something wrong with your computer's controller chip. I haven't had a drive become sluggish like this unless the system was infected with a ton of malware.
Was having a myriad of problems, the latest was a windows disk error which prompted me to get everything off it (I trying to crash plan it all but that seemed to start causing problems too...)
Started again with a clean install of windows on a third drive, and all pulling it all off on to a USB HDD as even the c drive has shown some errors in HD tune. Fully suspect the board or something to be the problem but I don't have access to another rig at the moment - unless its possible to connect it to a laptop some how?