How can I transfer the (entire) contents of one external hard drive onto another?

I recently had a moment where I feared something was wrong with my Lacie external hard drive. Thankfully it turned out to be a false alarm, but it got me thinking; what should I do if my Lacie really does conk out? I rely on that Lacie a lot for backups.

I have a second Lacie that I've never used, but I realized that my computer only has one Firewire port, and there are some things on my active Lacie that are not on my computer that I'd rather keep. Plus, given how much is on my first Lacie and on my computer, I'd rather not dump all the contents of the first Lacie onto my computer while I unplug the first Lacie and plug in the second.

My iCloud account is in limbo (don't ask), so that's out of the question. Is there some way I can plug in both Lacies at once so I can just drag everything from one to the other (like a hub or whatnot)? How can I transfer the contents completely from one to the other without god-knows-how-many extra gig sitting on my computer?