Is your MBA a USB3 model?
You are likely going to want a microSD card that uses MLC NAND - where as most SD cards use the cheaper TLC NAND, they are often much slower with their write speeds and tend to have inferior endurance (and I would argue many have inferior reliability.) It may also be financially justifiable to go for at least the 64 GB size, if not 128, if you are already using 20 and only have a 128 internal drive.
The
Transcend Jet Drive Lite* is a one-piece model that fits flush with the MBA's unibody construction and has both the adapter and the flash storage in a single product. IIRC the smallest available size is 64 GB and it goes as large as 256 GB. It uses MLC flash memory, and so its working speeds are comparable to some 5400 RPM hard drives. This is the simplest solution as you only need one product, and this product has been proven to be reliable when used for longer-term storage. Like many other MLC Transcend products, it has a lifetime warranty.
Additionally, other third party Makers offer some flush adapters, such as the Nifty Drive (a product I own and am very happy with.) These adapters require microSD cards to be purchased separately.
The two standalone mSD cards models I like for this sort of use with an adapter are the
Samsung PRO* and the
Transcend Ultimate* - both of these microSD cards also use MLC flash memory as well. Their write speeds are higher than the JetDrive Lite, although the adapter being used could impact performance.
Other models made by Transcend and Samsung use TLC NAND (such as the Samsung EVO and Transcend Premium lines). These cards often have slower real-world speeds, especially in the smaller 32 and 64 GB sizes. For reference, my 32 GB Samsung PRO microSD cards have a write speed that is about five
times that of my 32 GB Samsung EVO microSD card. For a product you will use as a second hard drive, the faster working speeds can come in handy.
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