I had a (Chinese) 8GB class 10 sdcard that tested fine with h2Testw but gave mmc0: Timeout waiting for hardware interrupt - cmd18 in the Raspberry PI.
So it was unusable to boot the PI but I also found that I could not get rid of the partioning using windows. All methods I tried only formatted the 56MB first parttion. On my Windows 8 PC disk manager showed only one partition.
Adding check_mk to our Nagios server was easy so I went on to install the check_mk agent on one of our servers.