Personally I don't recomend using more than 1-2 USB devices at a time. It's okay to have them physically plugged in at the same time - but if they are all powered on you are going to get problems because the power supply inside the computer can usually only handle a maximum of 2 power-sucking USB devices.
In the case of the USB hard drive it is best to use that on its own as that one probably uses the most power.
The Ipod pretty much acts like a hard drive so again use it on its own - don't try to use at the same time as the USB hard drive otherwise errors will occur - if you need to copy files from the Ipod to the USB hard drive copy them onto your Computer's hard drive (usually C drive) and THEN switch off the Ipod, turn on the USB hard drive and copy from the C drive onto the USB hard drive.
Card readers usually function okay even when another USB device is switched on - they seem to use the least power. But again copy files to the C drive FIRST and then to USB devices. Moving files between external devices can be problematic unless you do this.