Only two weeks in, but the memory card seems to working great.
I got this card for my kindle as it was specifically recommended by Amazon, and my kindle came with only 8 gigabytes of storage, which is ridiculously insufficient for any modern complex operating system expected to accumulate data. Even worse, it's not really 8 full gigabytes. (Also, I got tired of being nagged about running out of space after taking only a few dozen photos.)
Computers like numbers that are in powers of 2, and work best with processors and RAM that are built on powers of 2, which is why you usually see numbers like 8, 16, 32, etc. and not "strange" numbers like 10, 21, 43, etc. For this reason computer systems have *traditionally* used 1024 (2^10) as a base when reporting storage usage in kilobytes, megabytes, gigabytes, etc.
When it comes to a physical storage medium, however, capacity does not have to be a power of 2; it can be whatever number of storage positions will *physically fit*, the only real requirement being that the computer processor can count that high.
Manufacturers are notorious for cutting corners in manufacturing wherever possible then hyping up their products. In the case of the kindle, Amazon used 1000 (10^3) as the base of its "8 gigabyte" claim and aimed for a physical arrangement that would guarantee *at least* 8,000,000,000 bytes of storage (1000*1000*1000*8), over half a billion bytes short of 8,589,934,592 (1024*1024*1024*8). Not surprisingly, SanDisk did the same thing with this less-than-32-gigabyte memory card, as this deceptive practice has become an industry standard. But I digress a bit, my point being that an 8-gigabyte kindle affords you with MUCH less storage space than you would expect, necessitating a memory card for expanded storage, especially since the kindle's operating system uses a LOT of overhead, and every application you install also wants to use a lot of storage.
When inserting your new memory card (or removing it) it is best to do so while the device is turned off. Mobile devices are finicky when it comes to "hot swapping" memory cards, often refusing to read the card, or even worse, damaging the card and corrupting data.
After you've inserted the card and turned the kindle back on, go to Settings -> Storage, and move all the option sliders to the right to tell your kindle to utilize the memory card as much as it is willing to. Unfortunately, some features will still waste space on the internal storage, e.g. screen captures and blue tooth file transfers will NOT be saved to the memory card.
To "move" my kindle books to the memory card, the easiest way was to delete them then re-download them. To move other personal files to the memory card, I connected my kindle to my PC and used Windows Explorer.
From the previously mentioned settings menu, you can also move *some* applications to the memory card. Before and after the move, the amount of storage used on each disk is displayed. Moved applications will still use *some* space on the internal storage, and funnily enough, moved applications use more TOTAL storage than when they resided solely on the internal storage.
One application I did NOT move was ES File Explorer, as this is a system-critical application (due to the kindle's shortcomings).
It should also be noted that the kindle's crappy built-in file browser CANNOT locate the memory card. The files *are* there, but you will NOT find them without a proper file browser like ES File Explorer. Also, some programs (such as the red Notepad) will not be allowed write permissions to the memory card!
A few days ago my wife turned on my kindle and it updated itself which took over half an hour, which seemed unusual because when I observe updates they tend to run much faster. Afterward, my kindle stopped recognizing the presence of the memory card. The solution was to turn off the kindle, remove the card, turn the kindle on, then back off, then reinsert the memory card, then turn the kindle back on.
The card is working great, and is less than 3% full while my kindle has less than 1.5 gigabytes free.
Update 2018-04-23
Last night just before I went to bed, my kindle battery was running low. I was too lazy to go find a charging cable, so I just turned the kindle off.
This morning I turned it back on, with only 12% battery power remaining, and found that several program icons were dark. When I tried to launch one of them, I was told that the program couldn't run because the memory card could not be found! I tried removing the memory card and restarted the kindle like I described in my original review, but the memory card could STILL not be found.
I took the memory card out and connected it to my desktop computer and confirmed that it was in perfect working order. I then put it back in the kindle and connected the kindle to my computer, letting the charge process turn the kindle on. The darkened icons were still dark, and I saw a flash of the memory card icon at the top-left of the screen. I opened ES File Explorer which located the memory card and showed me its files. I went back to the home screen and none of the icons are dark any more.
I guess my piece-of-crap kindle doesn't want to exert any effort accessing the memory card if the battery power is red-lining when the device boots up.
Chris