GLi implements USB a bit different than standard OpenWrt does, and they may offer you another way. I think it uses UPNP, but not sure.
In standard OpenWrt you need usually three things
USB Basic Support - USB Basic Support [Old OpenWrt Wiki] (USB is not only for storage, you can run Wifi and 3\4G adapters)
USB Storage Support - USB Storage [Old OpenWrt Wiki] You will need to select one or more file systems (KMOD-FS-…)
Samba (or similar depending on your host client environment) - Samba [Old OpenWrt Wiki]
You will not get any of the nice GUI features that you get in GLi, meaning all file management is client based. You can configure all things through LuCi, though you may need to set some permissions using Putty.
I did not need to do anything special to get the SD card working on my “other” device, but if you have problems try configuring a USB thumb drive to validate all is good. Never got NTFS running, and mostly use FAT32 (VFAT) which is exactly the opposite of what works in the GLi config\firmware. Also was unable to get EXFAT working in the Luci GUI, though I think it was working in CLI. I reformatted my 64gb micro-sd to VFAT. If your not on Windows you should be able to use EXT3 or EXT4 which I think will offer the best performance.