I'll start off by recognizing that the OP said they were not concerned about price, but then in a comment said they ruled out using an SSD because of price. I think the OP has mistakenly assumed that a spinning disk or flash drive will be cheaper than an equivalently or better performing SSD.
They're wrong, especially given they only need about 64 or 128G, an SSD is the fastest and most economical option. I'm interpreting the OP's need as "I need an external drive for gaming on a tight budget".
The tl;dr version is that spinning disk performance isn't in the same league as flash drives and SSDs. A fast flash drive in the size you need will cost you $140. Instead, for $60 you can buy a cheap SSD that will perform as well as that flash drive, for example the Kingston SSDNow or the OCZ Trion 100. For $75 you can buy an excellent SSD such as the Samsung 850 EVO.
Here's the details.
There's a lot of very cheap, but very slow external drives out there. It's hard to know what's actually inside the enclosure, external drives rarely provide performance information. Let's ignore them and instead focus on putting a bare hard drive into an enclosure.
A decently fast spinning drive will cost you at least $50, for example a Seagate Barracuda, plus another $20 for an enclosure. If price is no object, there is the WD VelociRaptor 600GB, one of the fastest mainstream spinning disk drives, but it's going to cost you $250 and it's going to suck power like nobody's business. You can get a 300GB refurb for about $60. At a max transfer rate of 145Mb/s it's only 1.5 to 2 times faster than the much cheaper Seagate Barracuda. You're just not going to get huge performance gaps with a mature technology like spinning disk drives.
What about a hybrid drive, a spinning disk with a small SSD for cache? In 2012, when SSDs were expensive, they made sense as an economic compromise. In 2015, when SSDs are affordable, they're dead tech. They're only useful when you need a lot of fast disk space on a budget. You can pick one up for about $80. While it will outperform the Velociraptor for small, repetitive, easily cached transfers (usually having to do with the operating system) which will be using the SSD, large or infrequently used files (such as loading a new level) will not do well.
Flash drive performance varies wildly. And you're going to want at least 64 GB to be safe (as a comparison, Battlefield 4 wants 30 GB) which jacks up the price. A fast flash drive of that size is going to be anywhere from $50 to $200 depending on how fast and how big. A Corsair Flash Voyager GTX 128G is a good choice. At $140, it can read three times faster than the Velociraptor.
Here's the "money is no option" lineup from slowest to fastest.
- Any hybrid drive
- WD VelociRaptor at $60 to $250 for about 140MB/s
- Corsair Flash Voyager GTX at $140 for about 450MB/s
Ok, what about an SSD? The OP ruled them out because of price, but there's a number of well performing SSDs for $60. The Kingston SSDNow or the OCZ Trion 100 will both perform better than the Flash Voyager GTX, especially at random reads and writes, at half the price. Bump up to $75-$100 and you can get the amazingly well balanced Samsung 850 EVO.
This is all moot because with a brand new, graphics intensive FPS game like Fallout 4 your bottleneck will likely be your video card. The OP said they have a MacBook Pro Retina Mid 2012 which means, fortunately, a dedicated GeForce GT 650M. This should be better than the minimum requirements. I'm making that assumption based on Battlefield 4 and Arma 3 having a minimum of the inferior GeForce 8800GT. But it's not great.
Before you go spending money on hardware for a game that doesn't exist yet and we don't even know the system requirements, try running a game like Battlefield 4 or Arma 3 and see how that goes.