@one1002 - Since this thread has been bumped, I had another go, and following your instruction, I finally got it to work - After 15 minutes! The only card which I had enough space on for the test to be able to run, was a sandisk class 2. This is obviously why it took a long time, but 15 minutes.

Seems to me like a very long time to read and write, then delete just 1GB, but hey, got it to work, so thanks for your help and also thanks to akih for the script.