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.