This seems like a "classic" interrupt Issue/problem. It generally seems to happen intermittent out of the blue!
The Processor is busy with someother task and suddenly a higher order interrupt comes, which create problems.
Generally, the workaround is a time out or a a reboot.
I could imagine that there is a collision of the interupts of the 2 cards. Hence, by only using 1 card the problem is reduced or gone.
These type of problems are a pain to debug. Since, the main problem is to find out under exact which circumstance the problem occurs.
I believe that it depends on what you want it to do!
For me it is primarily to download pics when away, rendering them into Lighroom. Then having a quick look and throwing away the obvious bad ones.
I assume that this is the typical dive vacation tasks :-) The rest is beer time/ Hammock time/Social time etc :-)
I believe that 4GB RAM is the minimum for these tasks. For more serious photo editing you will need more.
Rendering & moving over to PC is mainly a Processor task ( as long as the SD/CF card is fast enough).
Then it is just a question of processor power how fast the process will be ( clock speed and amount of cores).
It could be worth looking at the Atom/Windows8 Tablets that will come out next year. If you want a lightweight travelling alternative.
Will they have USB3 and can they render pics in Lightroom in a reasonable time ( rendering is mainly Processor power)?
I assume that Photo Editing is too much to ask!