I'm not able to write on my SD card with Win32DiskImager. I throws the error:
An error occurred when attempting to write data to handle. Error 121 The semaphore timeout period has expired
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This seems to be a problem with older (read: slower) SD cards. If I recall correctly, the creator had significant problems with cards that were lower than class 6.
You may be able to fix the issue by running
chkdsk on the card (requires admin rights). If that doesn't work, you'll probably need to buy a different SD card. At the very least I'd try a different card reader.