This is a memory leak, it happens when the source drive is faster than the output HDD where you store the files. Also, the problem can caused by a fast CPU as well. To summarize it, the buffer of the HDD fills too fast and causes a memory leak and displays the error above. To overcome this issue, while shrinking, press pause at every 20-30 % and wait for the activity of the HDD to finish. Resume the process, and everything should be OK.