The iBasso DX50 holds up to 2TB via microsd SDXC cards. These are not available yet however. Another option is to modify an iPod Classic 5th gen. by addiing a 240GB Toshiba HDD or even a 256GB CF card, but both these options are expensive and require research to DIY. Alternatively buy multiple microsd cards for a suitable player although this is pretty inconvenient, its easier just to buy another Classic. There isn't all that much consumer demand for the sort of capacity you are looking for as most peoples compressed music collection is often less than 50GB.
No product out yet that has something like 500GB-1TB of storage space on-board, but you can use a WiFi MP3 player with the WiFi HD or WiFi USB flash drive and get up to 1TB of portable, battery powered mobile MP3 playback anywhere you go.
Otherwise, any phone that has a data connection to the 7GB minimum Skydrive account for streaming playback anywhere you have service.
Some tablets have larger internal memory, to which you can add more through a MicroSD card slot.
eg. get the 128GB model and add a 64+GB MicroSD card.
Definitely quite sad - 10+ years after the first MP3s have launched and still nothing to hold my 1TB+ collection in a portable player >.<