I came across this issue today, got Ubuntu Server 12.04 LTS running on a DELL PC OptiPlex 380 box. for some reason I had to move this to a new similar DELL PC. now I took sata HDD out from existing PC and installed this to new PC.
having thought that hardware is exactly same so I won’t have any issue. but after starting up found out that Networking is not working at all,
ifconfig shows me only one local adaptor .
ok not to worry here is fix : how to fix missing network adaptor after moving Ubuntu server HDD from one box to another.
Edit “70-persistent-net.rules” files using VI or your faveroute editor.
and look for similar lines as below , at bottom. starting with SUBSYSTEM. you might have only one line but for some reason my file had two such lines.
Delete these lines that start with SUBSYSTEM or simply comment out #SUBSYSTEM and so on.
save file and exit your editor. for vi its ESC then type “:wq”
reboot system. and now it should just work as normal.
hope this will help someone.