|Actual conflicts between USB devices are quite rare. Usually the problem is they don't get along with ports that are not directly connected to the mboard, and/or they can't get enough current from the port they're plugged into, or there is an IRQ sharing problem. |
USB devices may not work correctly when they're connected to certain USB ports e.g. ports in a hub or on the front of a desktop case. If you have a desktop computer, you may have an IRQ sharing problem you need to fix, or your power supply may be starting to fail.
See response 3 in this:
Also - if you are using a USB extension cable, some of them have inadequate wiring and will not work properly with some USB devices.
In addition, all external drives connected by USB require that the port (or ports) they connect to can actually supply 500ma of current.
They often don't work properly in a port in a multiple port hub that connects to one directly connected to the mboard USB port, or they work but only when they're the only thing plugged into the hub.
3.5" external drives are larger and always come with an external power adapter that plugs into the external case - that adapter must be plugged in and working because 3.5" drives plus their controller require a lot more than 500ma of current total. The hard drive requires both 12v and 5v.
2.5" external drives ("portable" or similar external drives) are smaller and usually don't come with an external power adapter that plugs into the external case. The hard drive and the external case only require 5v.
Recent 2.5" hard drives of any size draw up to close to but no more than 500ma of current while spinning up - the external drive enclosure's circuits don't draw much - so most will work fine when only one USB connection is made to the computer, if the USB port they plug into can actually supply 500ma (some laptop built in ports cannot; all the laptop PCMCIA and ExpressCard USB 2.0 controller cards I've looked at the specs of so far can only supply 500ma total for all the USB ports in them, unless the card has an 1/8" jack for connecting an external power adapter to and you connect one).
Older models of larger sizes of 2.5" hard drives draw more than 500ma - those must have two USB connections at the computer end, or if the case has a 1/8" jack, one USB connection and an external adapter can be used. Because of those older hard drives requiring more current, you often see that an external enclosure is not for xxxgb drives and above, but if it's a recent larger drive the external enclosure will handle it fine. If in doubt, look up the specs for an older model. or read it's label.
Some 2.5" external drives or enclosures come with a Y USB cable, or 2 cables, so you can connect to up to 2 USB ports on the computer end. Some have an additional 1/8" jack on the case you can plug an external power adpter into it if you need to.
If your 2.5" drive won't work when you connect only one USB connection to a computer and it has only one plug on each end, when it is connected to a port it should be able to get 500ma from that's directly connected to the mboard, if the case has no 1/8" jack for a power adpter, you need to get yourself a USB Y cable to suit.
If you get a 2.5" external enclosure or prepackaged external hard drive, I recommend you get one that has 2 cables or a USB Y cable, or that has one USB cable and a jack on the case for a power adapter.
E.g. Vantec - has a USB Y cable, one of the two plugs on the computer end is pass-through - you can connect other USB devices to it.
Similar applies to firewire connected drives.
eSATA connected drives don't have the current problem, that I know of.