While working on the GX1 I encountered an error with the 'natural3' hack. The mp4 modes shut down with the complaint that my card could not keep up with the data. It was phrased far more cryptically :# but That was my inference!
I looked for online solutions, deciding that a U3-class card should manage if others' stories were correct. A few 32G cards would serve for under $30.. but I decided to look locally just in case
After a couple of failures I visited my local Kroger store (Fred Meyer here in the Pacific NW) and hit a deal! The 64GB Sandisk Ultra Plus was on sale for just over $32. I brought it home, formatted it - and the message remained.
Now that I've shifted to a gentle version of the Galaxian hack I have no issues, and a large and fast card to hold my data. Some times I do catch a break!