Sometimes USB HD does not appear connected in desktop
I am using a new iMAC and in general terms I am quite happy with it. However, I have connected to it, through a D-Link 4 port hub, with external power source, two USB HD, both of which being 1TB each, an Iomega with its own power source and a Toshiba, which has a double pronged usb cable, one of which is to draw the extra needed power to run this HD (if and when required). As I said, both are connected to a D Link hub, which in turn is connected to one of the rear USB ports in my iMAC.
However, at startup, I sometimes discover that the iMac did not detect the HD, although both iMAC and the hub in question are connected to the same power source distribuitor.
Does this mean that the USB port is mal functioning? Do I need any special upgrade software (although Software Upgrade says all is in order), or is it the D link? I get over the problem by disconnecting the D Link USB cable from the rear of the iMAC and plugging it again to the same USB rear port.
Would you guys recommend a different setting?
Thank you in advance for your help.
iMac, Mac OS X (10.7.2)