XG Series User's Guide Chapter 6 Managing the Device
Actions When Firmware Update Failes (Backup Firm Function)
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6.4.2 Updating the Firmware
You can perform firmware update by transferring the firmware from an FTP client or a compact flash card.
Reference
"6.3.1 Updating Firmware Using FTP" (pg.568), "6.3.2 Updating Firmware Using CompactFlash card" (pg.570),
"6.3.3 Updating Firmware Using USB memory" (pg.572)