I had severe issues using a brand new SD card in a brand new unrooted Galaxy S5. However, I now found through test that it was not the card.
I tried many things, formatting on my computer, formatting on another phone, etc etc. Nothing worked. I then tried to use the card on other devices, which worked fine! I then tried other cards, none of them seemed to work.
So I thought the S5 was defective. I did some searching online, and found that it actually is not defective. This is a feature of kitkat 4.4.2!
The card is perfectly usable, but under very restrict situations.
For example, reading is perfectly possible, but writing is controlled by kitkat. I found through some websites that Samsung decided to beef up security and at the same time make the use of external storage more clean.
Previously you could use any file browser to manage any file son any external storage, like an SD card. Although this gave great freedom, Samsung also seemed to believe that it is a source of security risk where rogue apps could read/write to folders they didn't create. They also seemed to believe this made management of files on external storage messy. To control this Samsung now allows properly installed apps to generate folders only in internal memory, and those same apps can only access folders they themselves created. The exception to this are factory standard default apps that come preinstalled on the device.
So my mistake was to use ASTRO file manager to move around a file. Once I read the issue online, I tried the same action with Samsung's pre-installed file manager, and to my surprise the file did transfer to the SD card!
To further clarify, I suspect this "feature" is limited to write activities. For example, backup to SD apps will fail to write apps to SD. Similarly, Airdroid cannot copy files from your desktop to the SD card, only to their own folder (and thus you need Samsungs file manager to move those files to the SD card).
What's interesting about this however, is that READ access seems to still be possible. ASTRO file manager can still read files from directories other apps made. Adobe reader can still read files on all kinds of folders.
The criminal spy app can still read and copy your banking files from your bank directory to its own directory for transfer to the criminal mind who will then plunder your account. So no, the security issue Samsung wanted to address has not been addressed.
What is my opinion about this? Well, I think Samsung made a nice attempt to improve security. However, they completely failed to address the biggest security risk, which isn't write access, but read access. And at the same time made it very painful and difficult to store large numbers of music, video and document files to the SD card. Because now you will either need to plug in your memory card into your computer, or move all those files via internal memory to the external SD card using Samsungs file manager, which of course takes much more time.
More info on the feature can be found here:
We could all ask Samsung to revert this "feature", but chances that they will, are remote since there are ways around this problem.
Of course, people who use many many apps/games will be out of luck, as they will be restricted to 16/32 GB.
Note to moderators: Can we make this a sticky for kitkat 4.4.2?