The reason that some readers (most, in fact) work with Macs and some (a few) don't, is that the ones that don't have small bugs in their firmware that makes them not fully compliant with the Mass Storage class driver on the Mac. Lazy manufacturers only test the devices on Windows, so they thrash out the bugs that affect working with Windows, but do not test on Macs so do not find the bugs that cause them not to work with Macs. I have bought many USB flash readers in my time and most have worked great, but a few have not worked reliably - apparently losing connection during long copies for example. The reason that some readers (most, in fact) work with Macs and some (a few) don't, is that the ones that don't have small bugs in their firmware that makes them not fully compliant with the Mass Storage class driver on the Mac. Lazy manufacturers only test the devices on Windows, so they thrash out the bugs that affect working with Windows, but do not test on Macs so do not find the bugs that cause them not to work with Macs. I have bought many USB flash readers in my time and most have worked great, but a few have not worked reliably - apparently losing connection during long copies for example.
2-Slot Micro USB OTG and USB 3.0 Flash Memory Card Reader for Windows, Mac, Linux, and Certain Android Systems – Supports SD, SDHC, SDXC, MMC / MicroSD, T-Flash [Black] $ 14.99 $ 9.99 SuperSpeed USB 3.0 for maximum performance.