What's wrong with using the mibox? I have a 4TB WD HDD connected to it.

Well if you dont care about streaming services like Netflix and Hulu, then I would go with a Raspberry Pi3. Image LibreELEC onto a MicroSD, insert, boots right into Kodi, which wont have any issues playing back whatevers on the hdd, plus it will be able to share out that drive to anything else on the network.