External SD card useless in LP? topic

One of the reasons I got this phone after having a G2 for a year was to be able to use external storage. Well I am now on my second 128GB sd card, which the phone is supposed to be able to recognize, but it's all but useless. When I inserted each card the first time the phone recognized it and was able to mount it and I was able to write to it. But whenever I tried to copy files using a file manager (I use Total Commander with root access) it will tell me to "select the root of the SC card to gain access" or something similar. So I do that, but it still fails to be able to write to it. Sometimes it causes the SD card to unmount and not be readable by the phone at all except in recovery. I'm on stock 5.0.1 from T-Mobile. Is this an android issue or a phone issue? The first sd card is totally corrupt - I can't even mount it on my PC to format it. I want to find out what the problem is before corrupting this one. The second one I just tried getting permissions using my QuickPic app to copy files over, and I followed the instructions to gain write permission and it seemed to indicate it was successful. But nothing was copied. Then suddenly the SD card was no longer mounted and the OS doesn't see it. I rebooted in recovery and it sees it there. I tried formatting the last one in recovery and that's when it got so corrupted that it was not usable. Any help here?