I just realized, that one of my flash cards has a very varying write speed. Some times 6-8 MB/s (which is not impressive for a SanDisk Class 10 that should write with at least 10 MB/s https://www.sdcard.org/developers/overview/speed_class/ ) but I have seen it down to 600 KB/s.
Update: I have written a post about my experience and research on microSD cards - don't buy to cheap .
I read on http://www.thriveforums.org/forum/toshiba-thrive-general-discussions/15040-sd-card-issues-problems-solutions.html that it might be due to file system fragmentation and a normal formatting helped. It did not. Now I got the around 600 MB/s some times, where the lowest speed before was 1.3 MB/s.
Has anyone knowledge in this area? The card is around 6 months old and have taken around 150.000 images + a little video which for the 64 GB card sums to around 15 total overwrites. Should I try to RMA it? Two of my other cards are still useable but only lets me write with 4 MB/S. Same type of card, same use but no video.
PS. My card reader is a Lexar, that reads with up to 80 MB/s from these cards and should do more for faster cards.