When you clone a root Ubuntu disk in VirtualBox, one thing that gets messed up is the network card definition.  This is because Ubuntu (as it should) uses UDEV IDs for the network device.  When you boot your new disk, the network device ID has changed, so it creates a new eth1 device.  Unfortunately, this conflicts with the VirtualBox network setup.  What to do?

Boot the box (no network)

Edit the /etc/udev/rules.d/70-persistent-net.rules

Delete the eth0 line and modify the eth1 line to be eth0

--------- Example OLD -----------

# This file was automatically generated by the /lib/udev/write_net_rules
# program, run by the persistent-net-generator.rules rules file.
#
# You can modify it, as long as you keep each rule on a single
# line, and change only the value of the NAME= key.

# PCI device 0x8086:0x100e (e1000)    <-------------------- Delete these two lines
SUBSYSTEM=="net", ACTION=="add", DRIVERS=="?*", ATTR{address}=="08:00:27:d8:8d:15", ATTR{dev_id}=="0x0", ATTR{type}=="1", KERNEL=="eth*", NAME="eth0"

# PCI device 0x8086:0x100e (e1000)     ---Modify the next line and change eth1 to be eth0
SUBSYSTEM=="net", ACTION=="add", DRIVERS=="?*", ATTR{address}=="08:00:27:89:84:98", ATTR{dev_id}=="0x0", ATTR{type}=="1", KERNEL=="eth*", NAME="eth1"

----------------------------------------

--------- Example NEW -----------

# This file was automatically generated by the /lib/udev/write_net_rules
# program, run by the persistent-net-generator.rules rules file.
#
# You can modify it, as long as you keep each rule on a single
# line, and change only the value of the NAME= key.

# PCI device 0x8086:0x100e (e1000)
SUBSYSTEM=="net", ACTION=="add", DRIVERS=="?*", ATTR{address}=="08:00:27:89:84:98", ATTR{dev_id}=="0x0", ATTR{type}=="1", KERNEL=="eth*", NAME="eth0"

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