External hard drive problem

Hello,
I have a problem with my external hard drive. I have used it normally for over six month. recently i discovered i couldn't read my files or write to the disk. The disk reveals enough free disk space and my widows xp system reads the disk. when copying files to or from the disk, an error says " windows delayed write error". I have tried basic disk commands like chkdsk without success. Same problem happens when i use the disk on vista.

If this is a hardware problem, please advise me as i have valuable data in the disk.

thanks