An IRQL NOT LESS OR EQUAL error is usually caused by a hardware driver or IRQ conflict. An IRQ is an interrupt request where a device requests time from the CPU. When different devices compete for the same IRQ, there are operating conflicts.
One of your devices may be in conflict with another device on your system. There is no one specific fix for this error. You will need to narrow it down by removing drivers or devices until it does not do it anymore. I would first try uninstalling your printer and card reader completely and then trying your external hard drive. Remove the software from Programs and Features and check Device Manager to make sure it is gone.
You may also be able to avoid removing devices by disabling them in Device Manager instead and then try to connect your USB device.
There is also a chance it could be related to a driver update for your sound card, video card or other device that you may have performed or even Windows Update performed.