This thread may become obsolete soon, in view of the availability of 1TB USB flash drive.
With a portable DAP with USB OTG functio, you may be able to add 256GB-1TB of USB flash storage now.
Kingston DataTraveler HyperX 3.0 USB flash drive offers up to 1TB of USB flash storage:
1TB USB flash drive is available now for $1260:
512GB for $530:
I said "you may be able to ...." as I haven't tried them myself. Kingston's web pages indicate that those USB flash drives are exFAT-formatted, backward compatible with USB 2.0 and will work for Windows, Mac and Linux. Assuming that no special driver is needed (as Kingston does not indicate such), I am speculating that those drives may be reformatted as FAT32 and work with any portable DAP with USB OTG function.