It's not so much what can be written to an SD card as what apps can read and write where.
Prior to the change, you could easily move files around with a good app like EStrongs File Explorer (aka ES File Explorer).
Check out "ES File Explorer File Manager"
https://play.google.com/store/apps/details?id=com.estrongs.android.pop
Android apps are controlled by their security permissions. Check out
http://androidforums.com/android-ap...explained-security-tips-avoiding-malware.html
The new change introduced what you can think of as an override to one of the permissions.
Before, decent apps would make their own folders on your SD card, keeping things nice and organized.
And you could shift things as you needed, say with ES.
Or you could let apps work and store where you pointed them.
No more.
Suppose you take a picture, and the camera app writes it to the card.
Now take Snapseed (great photo editing app), edit the picture. Before, you could save it back to where the picture came from.
Not anymore - with the change, you can only save it to the Snapseed folder.
If you're what my friend Medion (who taught me what's up) calls a power user, using multiple apps to manage your media, it's going to be a pain in the neck.
The non-rooted workaround is to set up media folders under the folder that ES owns, copy everything there using ES, and then let your media apps read from there. But read-only.
A lot of people don't use file explorers or multiple similar apps. They're probably not going to notice the change. A lot of people will notice the change and be OK with the workaround strategy to copy things to central locations by a file manager and then other apps get read-only access to that. A lot of people just use apps from Google, or pre-installed by the phone manufacturer or carrier. Those apps don't have the the new restriction, people that just use those won't notice.
The rest of us have no intention of going along with it.
You have to change one parameter if it's set and you have to be root to do it.
Check out "SDFix: KitKat Writable MicroSD"
https://play.google.com/store/apps/details?id=nextapp.sdfix
Read that app description carefully, it tells the whole story precisely.
That's one of the two apps that I said to bookmark earlier. Do it if you haven't. Because of the common words in the name it's really hard to search for.
That permission change was made for security reasons. People didn't like the idea of apps being able to get full storage access. Google responded.
This really is going to affect everyone.
The same change is coming to your internal storage. Today it's an SD card issue, tomorrow it's everything.
A lot of apps got broken when the change hit. A lot of confusion stems from that.
Big change like this, Google usually makes the changes to the developer kits first, give everyone time to adapt their code before it goes live.
This time Google pushed out the update and the developer kits changes at the same time.
And people affected got really pissed off. I don't blame them, I would have been too.
I do wish though the spec topping train would not be the main driver for products, if performance is dinged. Seems apparent here that performance was compromised due to the display. 
