I'm far from an expert so this is my personal opinion...
Depending on how much data you are transferring, a Thunderbolt 2 drive could be a wise investment or a completely unnecessary expense, IMO.
- What size external SSD do you think you want? (500 GB, 1TB, 2TB?)
- Are you transferring many smaller files or fewer larger files?
- How much data would you estimate you transfer per-day? (in GB)
- Is cost or longevity more important?
Thunderbolt 2 can support stupidly fast PCIe SSDs - but often at a prohibitively high price (think several hundred to a thousand dollars or more depending on the specifics.) This comes with the benefit of being TRIM-capable, but usually at the expense of being relatively large and often requiring an external power source (so it is not very portable.) Of the limited models that are bus powered, they are extremely limited in capacity and are still quite bulky because of heat dissipation requirements. Speeds approaching 1,500 MB/s are quite possible.
USB 3.1 gen 1 (what you have, which was formally called USB 3.0 SuperSpeed) can support fast SSDs that use the SATA interface. USB on the Mac does not support TRIM, although the necessity is somewhat debated (I'm a believer in it.) These SSDs are bus-powered and can be as small as the size of a business card - so they are highly portable. They are also much more affordable. Their transfer rates will be more in the ballpark of 250-450 MB/s.
If you go with a USB SSD, you have two options: purchase a completed external USB SSD (such as the Samsung T3, T5, or ADATA 700) or purchase an internal SATA SSD (such as the
Crucial MX300,
Samsung 850 EVO,
Samsung 850 PRO, or Transcend 370) and a USB-C enclosure (such as
Oyen Digital USB-C enclosure, this
Orico USB-c, this
Aukey USB-C [all of which you can use on your current system with a USB-A-->USB-C cable and I'd argue one should favor USB-C because it is a better connector than USB micro and more future proof]) separately and then put the internal SSD in the enclosure to make your own external SSD (the process is easy.) Sometimes you can piece together a SSD that uses 'better' NAND (the memory inside the drive) and comes with a longer warranty for not a whole lot more than the price of a pre-made model. For example, the fully completed 500 GB
Samsung T5 is priced at $200 - it uses 3D TLC NAND and has a 3 year warranty (it will probably be a fantastic SSD.) The
Samsung 850 PRO internal SSD is priced at $225-240 depending on the source - it uses the desirable 3D MLC NAND and has a 10 year warranty. For some, the relatively small price difference might be justifiable for what they get in return. Many pre-made SSDs are more physically compact in size than what you would get from piecing together an internal SATA SSD + a USB enclosure.
For reference, I would estimate that a standard 5400 RPM 2.5-inch (laptop-sized + bus-powered) HDD (spinning disk drive) can transfer data around 70-100 MB/s (plus it has latency, which SSDs do not.) Some 7500 RPM 2.5-inch HDDs are closer to 90-135 MB/s. Some 3.5-inch desktop-class HDDs (which require their own dedicated power source) are much faster. Some enterprise class 7500 RPM HDDs can transfer data upwards of 270 MB/s. This might help give you an idea of whether or not a USB-SATA SSD will meet your needs, or if a TB2 SSD might be a better match?
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If your current external hard drive is older or smaller (which I assume is an external HDD, or spinning disk drive), then I would recommend purchasing two different physical drives: an external SSD and external HDD. (If your current drive is large enough and young/healthy enough, you could use it as well.)
The first drive would be the external SSD where you would store your files on that you may not keep a redundant copy of on your MBP's local drive (and vice versa.) As the external SSD and internal SSD would both have unique data on them, on their own this is not a backup solution given, if one fails, you could lose important data and you might have a rough time getting set back up once the failed drive/system is replaced.
Enter the second drive. This other drive would be an external HDD that would be configured to back up both your external SSD as well as the local SSD inside the MBP (either using something like TimeMachine, Carbon Copy, Cloner, etc.) This way you would have at least two copies of your important files.
However, even that isn't ideal if you have files you really cherish, given both mediums would be prone to environmental damage (e.g., flood, fire, power surge, theft) since they are usually stored essentially in the same location, plus HDDs and SSDs fail all the time, and while a simultaneous failure is not common, it's also not unheard of. If you also use cloud storage, this third offsite backup gives you even better data integrity by accounting for many causes of data loss that the above external SSD and external HDD cannot on their own.
(If you already have a backup plan in place, I apologize for the redundancy.)
