When setting up a new laptop, Setup Assistant doesn't see all the data

I'm trying to set up a new Retina MacBook Pro, which is running OS X 10.10, by transferring data from my old MacBook, which is running OS X 10.8.6, via Thunderbolt. My old laptop is in Target Disk Mode instead of using Migration Assistant due to the OS mismatch (the new machine wouldn't let me migrate with Migration Assistant). As far as I can tell, this all matches what Apple says should work. However, I have ~350 GB in my home folder, and my new laptop can only see ~50 GB. What's going on, and is there any way to fix it?

A few things:

1. 
   

   I tried copying my data once already, but almost nothing got copied (I think even less than the ~50 GB).

   
   


2. 
   

   I can't update my OS on my old laptop, because I don't have enough space.

   
   


3. 
   

   I could try restoring from a Time Machine backup – it seems to see everything that way – but I don't have the right cables, so (until I can get those) I'm looking for a laptop-to-laptop solution over Thunderbolt. (I have too much data to use Wi-Fi.)