@kristiaan_b
Lets start by listing all the packages you ADDED for USB support, including the FS (file system packages). Also include your /etc/config/fstab file.
How are the 2 media formatted? Please consider formatting the SD as FAT32 so it is VFAT compatible, at least for now. It has the least issues with file security, and will be the easiest way to validate the HW is working. If it is formatted as EXFAT I do not think you will see it.
Please confirm you have the 3 firewall rules, though I suspect that if your USB thumb drive is working then this is not of concern.
Check your kernel log (Luci) by searching USB. In my case there are 30 or so log lines that indicated there are multiple USB (USB 1.0, USB 1.1 related to ehci). Post what you see here too.
I do not think you can compare the GLi usb implementation to the (more or less) OpenWrt documented process. There is no fstab in GLi. VFAT does not work and NTFS does.