+1 for this feature. I'd love to be able to keep track of where various copies of my cards live, and it sounds like my collection isn't nearly as extensive as some of the others requesting this feature. Given the other optional columns, it feels like it'd be pretty straightforward to implement too. Right now I'm still deciding if I want to catalog my collection here, or at echoMTG, or somewhere else. Unfortunately they all seem to have a single big flaw, and no location tag is the flaw at Deckbox.org 
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