it's already working. these are the steps to follow: 1.introduce a low-capacity flash drive on raspi making the system folders. 2. Copy the partition with a partition manager program into the new flash drive. 3. Expand the partition. Now I have a Sandisk 256Gb flash drive fully working on my raspi 3!

I have bought this usb on amazon https://www.amazon.es/gp/aw/d/B00YFI1A66/ref=mp_s_a_1_1?qid=1459898390&sr=8-1&pi=SX200_QL40&keywords=sandisk+256gb&dpPl=1&dpID=313u-coHKiL&ref=plSrch And not working, I have tried to format on fat32 exfat and ntfs and nothing, detected but not reading roms.