windows - WD My Passport detected but not assigned a drive letter

I have a WD My Passport external drive which used to work fine on my Win7 x64 box until recently. Since the past few days, windows would not detect my external drive. However, Storage under disk management lists my external drive(which means windows can see my drive) but without a drive letter. I am not able to assign a drive letter(the option is grayed out). I installed EASUS Partition Manager which allows me to assign a drive letter, but fails to do so when I apply to save changes. What do i do to assign a drive letter to the drive or make windows to detect it as of old? The external drive works well on Win7 x32(dual boot, same system) and win7 x64 on another box. Update: cant comment from Opera Mini 6 @zeke the drive works absolutely fine on other Win7 boxes and on the dual boot win7 on the same system.