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The first thing I would do is take it out of the external case and install it as an internal drive.
If it works fine then you have ruled out physical error with the drive, so it's now to the external case.
As it's worked fine with the plextor then it could be the increased speed of the samsung drive.
Try with slower media (or CD's) and see if it burns fine, or manually drop the burn speed.
you can also unplug some other stuff from the other USB ports and see if the extra bandwidth helps.
Another option is to unplug the drive and put it into a new USB port, this will make Windows see it as a brand new device and may clear some problems.
If all else fails then maybe this isn't a good drive for your external case (some drives are known not to work too well with certain chipsets) and would be better keeping this one internal if possible.
Hope this helps, let us know how you get on