I experienced the same with wifi drivers ( firmware-iwlwifi) which weren't automatically recognized although being located in the root directory of the USB device. How do I include the rtlwifi drivers on the same disk as the debian ISO so that the installer can find them? Or alternately, how do I make a 2nd USB disk that the installer can recognize? I also tried placing the files on a separate USB, but the Debian installer couldn't find/detect them (and unfortunately, the installer isn't designed to give any info on why it failed). But I can't mount the partition, and receive error mount: wrong fs type, bad option, bad superblock on /dev/sdc3, missing codepage or helper program, or other error So (following various instructions found online) I then tried to create a second partition in the remaining space on the drive (partitioned/formatted as VFAT), and move the files there. I wanted to put them in the /firmware folder on the install disk, but the it's not writable. I extracted the drivers from the realtek folder in the firmware-nonfree package.

The installer works fine, but my laptop requires proprietary wireless drivers (RTL8192) to get internet during the install. I downloaded the Debian amd64 netinstall ISO and placed it on a USB stick using dd if=debianblahblah.iso of=/dev/sdb

Realtek wifi drivers .deb