I have an external CD/DVD player/burner attached by USB but it is not being recognised by my iMac
How can I resolve this problem?
In order to get real help you must be more specific. What iMac, which version of Mac OS and what DVD drive. Also, does the DVD drive have it's own power supply and is it connected directly to the Mac or through some type of hub. Finally, have you checked with the drive's manufacturer to verify it is compatible with your Mac?