i will update this guide whenn i'll find new circuits!
here are some circuits which i use very often (i'm blacksmith). they are meant for farming ores of mithril, dark iron, thorium and truesilver:
alterac: for truesilver (red) and mithril (yellow)
burning steppes for dark iron:
winterspring for thorium and sometimes mithril:
the old copper farming guide for HORDE:
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the new copper farming guide for ALLIANCE:
first go to ratchet and follow the yellow line
after this, go to durotar, follow the yellow line again until you arrive the red arrow and follow this on back to ratchet (the map above of the barrens continues the red line)
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arathi for iron:
the red line is for horde, the blue one is for alliance