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Have heard a bunch of questions recently about removable USB drives being not detected, or being detected incorrectly when moved from one laptop or desktop computer to another.
Please give us as much specific information as possible, regarding your system. Which My Book external HDD you have, size, if it is connected by a USB connector, if you use an external power block or if it gets power from the USB connector, and initially if the data files were copied from a Win-XP computer or Win-7 computer. Also if this problem has occurred after attaching it to a different computer.
Also what computer OS was used to copy the files to the My book, and if it is the same computer and OS you are presently having trouble with?
Finally, on your laptop, with the MyBook attached, go to Disk Management, to see if it is recognized by the OS there. If so, in the lower graphical section, list for us what is displayed in the Drive Status section (Disk 1, Basic, sizexxx, Online) and below the Volume Status colored bar area (volumeName, driveLetter, sizexxx, Healthy Primary Partition) or what is listed.
It's a lot, but with that info, one of the group will figure out what's going on.