Because my old small HD (WD Element 2.5" - 2TB) is sometimes difficult to access for all my music files due to corrupt directories, I replaced it with a new one and now I have enough space again. To make it work with MacOS and Windows I have formatted it exFAT (instead of NTFS).
Western Digital 2.5" disc My Passport 4TB
connection: USB 3.0 A (+ C-adapter)
weigth: 210 gram
Bol.com: €107
Also bought a fast USB 3.0 Hub:
Nexibo including power supply
(ideal for external HD's)
4 ports USB 3.0 (5Gbps)
on/off switches with leds
Bol.com: €25