Intermittent slow USB copying

I have three USB ports on my 2009 17" MacBook Pro and connect a variety of USB devices; e.g. camera cards, HDDs etc. Sometimes, I start to copy and (as is happening as I type, Finder says that it will take 6 minutes to copy 3.66GB. However, a few minutes ago, I started to copy the same files and I abandoned the operation after about five minutes because Finder said that it would take 2 hours and had only copied about 100MB. After unmounting the SD card and reconnecting it to another port, everything seemed to work perfectly.

The obvious answer might seem to be that a port is faulty but that isn't the case because it doesn't matter which port is in use - they all do it, it is the abandoning of the operation and changing of the port which makes the difference.