I have two external hard drive (500GB LaCies) that will not mount on my MacMini. The first one is connected by firewire, and the second is piggybacked onto the first. They don't show up in disk utility, but will mount just fine when I connect them to my macbook. They had been working just fine previously, I'm not sure what changed, but I would like to get them back to mounting on the mini. I tried repairing the permissions, nothing happened, any suggestions on how to fix the problem?