I have a wd 2500/032. That the doesn't appear on windows explorer when I plug it in.

SOURCE: My Computer does not show: Western Digital My Book Essential Edition 500 GB external USB hard drive

Well i got a simple 320 gb external hard drive, and since the first month i had problems, i solve the recovery issue by opening the hard drive, and find it out, the inside was broken, and it got unplugged. so plugged it back and got my info. if your data its more important than 200dlls or less break the thing open, because if you claim the warranty they are not responsible for your information.

SOURCE: WD Passport Hard Drive Malfunction

Unfortunately, you have probably lost all the files on the drive. It may be possible to take it to a PC Repair shop and have them run specialist software on it, although the clicking sound indicates a hardware failure, rather than software. You have my deepest apologies, but this does happen... although rarely.

SOURCE: WD ext hard drive cant access files - I/O error (think its bust)

Chances are the drop jarred loose something in the drive that on;y caused problems when the disks stopped spinning and started again. Here is a list of recovery tools that would do the job if there is something there to recover. http://www.snapfiles.com/Freeware/system/fwdatarecovery.html you will know within a couple minutes whether there is anything recoverable.
If it is that important you can send the drive off to www.OntrackDataRecovery.com and they should be able to get the files.

SOURCE: WD external Hard drive 320 GB hangs my computer everytime connect

all solutions are not helpful, if u just use safe mode then there is no problem with hard disk, its a problem of some services, u just try with safe mode.30-40% people has same problem.
girish