I have noticed that with some USB readers (not to mention other media devices, such as digital cameras) there seems to be an upper limit to the amount of internal storage they can support. When I insert low capacity micro sd cards (16GB or less) they work fine, however when I try 32GB or 64GB and the device cannot read the SD card. They are from different manufacturers, so I assume that this is the result of a more generic issue.
Question: Why aren’t some USB readers capable of supporting large capacity SD cards? Does it have to do with the computer architecture? It would be nice to know if there was a rhyme or reason to it all. Currently, it’s a bit of trial by fire; I'm never quite sure how to scale my memory due to the fact that I remain unsure whether it will be supported or not.
Answer
The general rule is that devices can use what is available at the time of manufacture.
They cannot be expected to know what future development may bring.
These cards are all physically identical from the outside, however SD Cards have several 'types', separated by Capacity - SD, SDHC and SDXC which are basically split by size - up to 2GB, 32GB & 2TB address limits.
They are also separated by Speed - from 2MB/s to 90MB/s - in a confusing array of icons... numbers in circles, from 1 - 10, U-speeds in MB/10 & V-speeds which at last give the true speed in MB/s, i.e. V6 ... to V90.
Consumer-level guide at How to choose an SD card
Overview at Wikipedia - Secure Digital
Comprehensive guide at the SD Association - SD Standard Overview
Simplest way to tell what any given device can use...
... read the manual, or see what icon is next to the slot [if it's anything other than generic].
There's no other sure way other than trial & error.
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