Windows 7 - Unable to detect external hard drive

1) Click Start > Run and type diskmgmt.msc

2) In Disk Management, right click on the drive which was unable to be detected by windows
     and select "Assign drive letter"
3) Goto MyComputer and you should be able to see the drive now.

If this doesn't work, please post your operating system details and screen shots of disk management.

--Pavan Kotharu