Problem Accessing CD’s and flash drive

For the past few months i’m bothered by a weird problem: once i place a CD in either my Samsung CD-Rom or my Plextor CDRW, or if i plug my portable flash-disk into the usb socket, the computer cannot read the data from either one. in fact, it doesn’t even show that there is a CD or that the flash-disk is plugged in.
However, if i restart the computer with a CD already in one of the drives or with the flash-drive plugged in, than there is no access problem at all.