If you are used to Windows, new to Linux then copying the error messages you get from a terminal will help.
If you are used to UNIX/Linux and have this puzzling problem then (it's not the obvious, do double check) :
You might like to look into something like "System Rescue"
http://distrowatch.com/table.php?dis...n=systemrescue which has a recent release. I think I've come across a bug on certain USB drives where they are read only somewhere (perhaps the NTFS 3G site) which is the reason why a recent release may help.
Another alternative would be to use a Disk Manufacturers utility that actually copies the bytes, without reading filesystems, but that would likely waste space on new disk until someone fixes the partition table; and they may not support the USB disks you have.
Or sometimes you may be able to open external drive's case and connect it termporarily to internal IDE system. Folk do this the other way round building internal drives into USB cases, so you may find that the disk on it's own is more compatible.