You didn't click on the Safely Remove Hardware icon in your systray before you unplug the external drives (USB, HDD etc). This could cause serious problems in it.
In XP, try to fix the disk error by:
Insert your external storage device. > Start > Run. Type cmd in dailog box and enter to go to DOS window. At DOS prompt, type the letter of the drive, say H: and enter. At H:>, type chkdsk /f and enter. Leave a space between / and f. Windows will lock up and run a disk check on that drive. You can minimize the DOS window and do other tasks then come back later for the report. If it's normal, type exit and enter to go back to windows.
Now try to open the device again. If it still doesn't open, chkdsk has failed to correct some fatal errors on the device. You might have to format the drive and start all over again otherwise you're just holding onto a dead piece of hardware.
Don't forget to click to release your external device before unplugging it from now on.
After thought: If Chkdsk fails, you can try to run partitioning tools to fix MBR problems in that device before formatting it.