Apple 27 inch iMac mid 2011 error code -3403D
Error code -3403D appears during an install of OS High Sierra.
Install stops and restart-D key results in -3403D.
iMac 27″, macOS 10.13
Question marked as
It indicates some sort of network issue. Try connecting the iMac directly to the router using an Ethernet cable for a faster & more reliable connection which bypasses hard to troubleshoot WiFi issues.
Do you have access to any other Macs from 2007 to 2018? If so, then you may be able to create a bootable macOS USB installer for 10.11 or 10.13 using the instructions in this Apple article:
Create a bootable installer - Apple Support