I've not used flash memory much so i'm not entirely sure that i'm interpreting the Microchip datasheet correctly for a SST26VF064B. The datasheet from their website says on page 24 - section 5.14 "JEDEC-ID Read (SPI Protocol)" that the manufacturer ID is 0xBF rather than the 0xDF that is being reported.
It might be nothing (because I can't interpret the datasheet correctly!) or it might indicate you don't have the device you think you have ...