I just found this thread. I can say it must be hardware issue. I have the NEW 64GB 95MB/s read, 90MB/s write microsdxc card from Sandisk. My Surface Pro 2 on Win10 will randomly not read the card. It knows the card is still inserted. The drive label changes from "64GB (F" (I named it that) to "SDXC (F". Normally if there was NO card inserted, the explorer window won't display a drive letter. If I take it out and reinsert it, it'll read the card again. I tried formatting card to NTFS and exFat. No difference. NO issues reading my 32GB 80MB/s read Sandisk card.