This happens on both (1) an Asus VivoPC VM62N-G050R with the supplied wireless Asus keyboard/mouse, running Slackware 14.1x64 (dual-booting with the native win8.1), and on (2) a generic whitebox with a wireless Logitech MK520 keyboard/mouse, running Slackware 14.2x64.
Everything works great on both boxes until I plug in an external usb disk, and then the keyboard/mouse hangs pretty badly: maybe 5-10secs between pressing a key and its display, or 5-10secs between moving a mouse and the cursor beginning to move in a jittery fashion.
But if I then plug a wired usb keyboard/mouse into any adjacent usb port, that works fine. Googling the problem coughs up several similar windows problems, but no relevant fix that I found.
A possible clue that I noticed is that the Asus mouse has a continuous little blue light on its bottom side. But as soon as I plug in the usb drive, that continuous light starts flickering, at maybe five hertz (five times per second).
Since the wired usb keyboard/mouse works, I'm guessing it's not a usb problem, per se. Only thing I can guess is that slackware's loading some module when it sees the usb drive get plugged in, and that module's interfering somehow with a module for the wireless keyboard/mouse. But I wasn't able to track down with lsmod exactly what's going on. And it's just a wild guess, anyway.