Can't read SD card mounted on Arduino Ethernet shield SD card reader

Hello,

I have 2 Arduino boards, one Mega, one Uno, and 2 Ethernet shield from the same brand, both including a SD card reader.
I am desperately trying to read some micro SD cards, one a 8Gb and one a 128Mb, both quick reformatted with my win PC using defauts (Fat, 1024 for 128Mb, Fat32 4096 for 8Gb. I created 2 test.txt and test1.txt files on both sd cards).
I tried CardInfo and others samples delivered with Arduino SDK, time to time I succeed to read the 8Gb directory using CardInfo, but launching a second time the sketch, the SD card is not more recognized, without having done anything else that pushing the reset button on the Arduino Ethernet shield.
To exclude any hardware defect eventuality , I tried the 2 Ethernet shields, both on the Uno and the Mega, with the same results.

Any idea will be welcome, I need to log some data for my project, this is a showstopper.
Thanks All.

Try using the SD card formatter here instead: