Yip,
For moment it's all right with the pyb-d. It seems that with litlle_fs, it's going a litlle bit faster...?
Otherwise, I tried to do the same with a pyboard. I can just format all flash mem. And then I have troubles with the use of sd card. So with a pyboard, the best is still original format for long datalogging session on sd card, I think.
Thanks.