I think you should get another 500gb drive and put them in raid 0, this will use the space of both drives (total 1000gb) and writes to both drives at the same time essentially doubling read and write speeds. You can use a 1tb drive and a 500gb drive but the total storage will be seen as 1000gb not 1.5 tb since raid will use the highest common capacity, which in this case is 500. The only downside is that raid 0 has no redundancy, so if one drive fails, neither will function and the data will be lost, so use it for things that can be gotten again like programs. So to recap I think you should get another 500gb drive.