Usb card readers not seen

Things were working well, then stopped.I have 3 card readers: one on Epson Picture Mate, 1 on HP 3380 and 1 dedicated installed in Tower. In Device Manager, they are in disks, but have a yellow exclamation point in Storage volumes. Not seen in Windows Explorer UNLESS booted with SD card in one card reader when that one and only one will be seen. If placed in card reader and rescanned in Disk Management, will appear in Disk Manager, but without drive letter and not be seen in windows explorer. Once empty card reader with drive letter seen when booted with msconfig started in diagnostic mode, but may have been with disk in reader. Have tried so many configurations that I am starting to get slap happy. Windows XP with ASUS motherboard. Bios seems to be OK in USB section. Any suggestions would be greatly appreciated. Thanks in advance.