I can’t connect more than one USB device

I’ve tried connecting a number of devices to my MacBook Pro but have found if I try to connect two or more, using either USB ports or a USB hub, the computer reports ‘disk not ejected properly’. Sometimes it fails to recognise the device and removes it from the desktop, or it stalls operation and I have to turn the computer off at the power button.

I’ve tried erasing the hard drive and reinstalling the OS. I contacted Apple Support and they suggested a SMC and NVRAM reset, and restarting in recovery mode. None of these have helped. The computer has been to an Apple dealer who tested the hardware but found nothing. Could a virus be present on the USB devices I’m trying to connect? Dominic Orosun

This has nothing to do with viruses or your hard disk or OS X. It’s a hardware problem. Something is causing the motherboard to lose the connection to one of your devices. This can be caused by too much current draw on the USB bus. I’ve heard of cases where unticking the ‘put hard drive to sleep when possible’ box in Energy Saver preferences helped, maybe because it avoids the brief current surge on the USB bus when the drives need to spin back up again. But in most cases it’s simply a matter of replacing your USB cables. Old or worn cables can have a broken wire with the ends loosely touching, or a dry solder joint at the plug end. The connection breaks and the Mac thinks the plug has been pulled. Sometimes the bad connection is on the device and a Thunderbolt port may work instead, if available. It probably isn’t a loose connection on the USB ports of your motherboard. You most likely only need to invest in new USB cables.

Putting hard disks to sleep when possible saves energy, but this has been reported to cause problems with USB devices on some Macs.