FTOS needs to see a FAT32 formatted USB stick. I have seen plenty of times where the stick inserted with FAT32 isn't recognised with a file system. If this happens just get the switch to format the USB stick with this command:
Once the stick is formatted copy the FTOS .bin file to the stick from a laptop and insert it back into the switch chassis.
Issue the command:
upgrade system usbflash: A:
After the file has copied to flash you issue the command reload to finish the process.
The process can take about 5 minutes. Please note in this example, I'm upgrading from version 8.x.x.x to 18.104.22.168 on a S4820T, in this case on reboot each physical port had to have a firmware upgrade from version 01.50 to 01.66 which took approximatly 10 minutes. Make sure you plan this into your downtime plan!
You can verify the firmware upgrade by issuing the show os-version command