I just got mine a week ago and could not believe that such an expensive piece of hardware could have such an issue.
I did some research around the web and found many complains with this device. Owners everywhere are facing the issue where they can read from the SD but cannot write to it. After reading some, I realized that it had nothing to do with "root access" (though you will need it). In reality, the fix provided above by "schalmers617" does resolve the problem. I went ahead and performed the fix with somewhat positive results. I can read/write with ease from the internal to the external SD port (no issues) and apps can also do so.
One issue does remain, if you connect the tablet as an external device, to your PC, and try to write to the external SD port, the transfer fails more than often. Copying straight to it is very, very, very unreliable. I was able to do such transfers by copying DATA to the internal memory and then using a FILE EXPLORER app to complete the move to the external SD.
Again, the fix works without issues. When I say that you "don't need root", I mean it. Once you complete the fix, you can "unroot" your tablet back with the issue solved.
I am very disappointed with Google and their QC. It seems as if they simply (and I am certain of it...) dumped a Nexus 7 2013 image unto the LG G Pad 8.3 GPE without doing any kind of QC Check; just a dump and SOLD AS IS... I mean "AS IS" because you can call Google Tech Support with the issue (which I did 2 times) and (their Tech Support is horrible) and they had no ideas or suggestion other than doing Hard Resets and Reformat.
The LG G Pad 8.3 GPE is an amazing piece of Hardware, it is... What is poor is "Google's Dump An Image And Sell Practice" with this outstanding tablet. What they have done has killed my confidence in their QC Checks. Though my issue is partially fixed, it worries me what will happen with a future upgrade if they simply copy over the OS from the Nexus and into the LG without first realizing that both products have different features...
In Short... to fix the issue, do "schalmers617". Have patience while you do it. It WILL work. I am currently using a SanDisk Ultra 64 GB microSDXC Class 10 UHS-1.
Since I have no PULL on this with Google..., I would like Android Central to take this up the ladder and ask for a fix. The LG G Pad 8.3 GPE needs a fix from Google ASAP. This will become a loud issue...
+2cents