Do you need any permission to read data from an sdcard? I found the write permission but nothing for read.
On a linux system cd to the sd card you may have a long path name to copy and paste. Try "sudo nano" or sudo vi depending on the editor you like. This enabled me to read files I could not open on the SD card with my standard gui based text editor.
For example in the GUI I tried to open passwd- file in /etc on my SD card. The icon has a little x on it. Leafpad my linux editor Failed to open the file '/media/UserDrake/28d3774e-c2bd-486f-bde0-3cbb17d2317c/etc/passwd-' permission denied. However if I use terminal cd / then cd to the just mentioned path using cut and paste because it really is long to type. I CAN see it with sudo nano "above path"