Some mainframe devices have Magnetic Reinforcements which protect them against EMP effects. When a device with Magnetic Reinforcements has more than 2 firewalls, then EMP effects break 2 firewalls instead of rebooting the device. Magnetic Reinforcements also protect EMP vulnerable drones from being destroyed by EMP effects when they have more than 2 firewalls.
Magnetic Reinforcements do not protect devices with only 1 or 2 firewalls, so in this case EMP effects reboot devices and destroy EMP vulnerable drones as normal. EMP effects targeting Barrier Guards and OMNI Protectors still disable their Heart Monitors, even when they have Magnetic Reinforcements and more than 2 firewalls.
Whether or not devices are protected by Magnetic Reinforcements depends on the mission difficulty.
The probability a device has Magnetic Reinforcements is independent of other devices. Additionally, Elite Barrier Guards always have Magnetic Reinforcements, even if they appear at mission difficulty 5.
Magnetic Reinforcements can be an unwelcome surprise in Endless modes or the extended campaign if you are relying on EMP Packs, Subdermal Tools or Fool to avoid hacking devices with high firewalls. Once Magnetic Reinforcements start appearing, these tend to become less useful.
There are a few ways to deal with Magnetic Reinforcements, depending on which EMP effects you use:
The upside of Magnetic Reinforcements is that EMP effects can now be used to break firewalls on devices you want to hack to control them, not just to disable them. This allows you to hack without using PWR, and is often more efficient than using Buster Chips — an EMP Pack III becomes like a better Data Blast, breaking twice as many firewalls in area of effect twice as large.