Hi, i have a similar problem, "SD card became unrecognized by card readers", but in different way i think.
I have a 512MB SanDisk microSD, and after some test of write and read it became unrecognizable.
I'm using it with Arduino Etehernet shield.
I'm trying to restore card with Panasonic SDFormatter tool but it does not recognize sd too...
Anyway, i tested this sdfat lib examples.
I try SdFormatter and result is:
Enter option: F
Card Size: 484 MB, (MB = 1,048,576 bytes)
SD error: f,1
And with QuickStart:
Card successfully initialized.
Card size: 507 MB (MB = 1,000,000 bytes)
Can't read the card.
Try another SD card or reduce the SPI bus speed.
The current SPI speed is: 4.00 MHz
Edit spiSpeed in this sketch to change it.
SdFat version: 20111205
type any character to start
init time: 4 ms
Card type: SD2
Manufacturer ID: 0X3
OEM ID: SD
Serial number: 1254761136
Manufacturing date: 11/2006
cardSize: 990976 (512 byte blocks)
flashEraseSize: 32 blocks
read MBR failed
SD errorCode: 0XF
SD errorData: 0X1
Can i do something?