WD 400 GB external hard drive doesn't mount after software update.

Welcome to the Apple Discussions!

First, have you rebooted your Mac? Try that.

Next, unplug the drive from the wall out let.
Wait about 10 seconds, plug it back in and see if it remounts on the desktop.

I've noticed with my own WD 500 Essential that it will go to sleep after 24 hours or so of inactivity and won't wake unless I reboot it.

Cheers!

Can you mount it, or find any useful info, from Disk Utility?

I'm having the same problem with my LaCie Big Disk. The only thing that has changed was a software update from Apple that was pushed out on Sept. 30.

Can Disk Utility see it? If so, can it mount it?

Have you tried having Disk Utility verify the disk to see if it finds any errors?

Disk Utility doesn't see it, but the LaCie firmware update could update the firmware.

c14user,

Looking back at your original post: "Partition Map Type: Unknown" is a little suspicious. I'm wondering if, while the disk was formatted correctly, perhaps the partition scheme was not changed from Master Boot Record when you first began using it. I made that discovering recently on a drive that I could no longer select as a Time Machine disk. If so you'll need to re-partition the disk either GUID or Apple Partition Map.

*_How Should a Time Machine Hard Disk be Prepared?_*
For Time Machine to work properly, the hard disk must be formatted “Mac OS Extended (Journaled)” and its’ Partition Scheme should be either GUID or Apple Partition Map.

Time Machine is incompatible with disks partitioned as Master Boot Record (MBR). Unfortunately, this describes nearly every hard drive you can buy because MBR is a Windows partition scheme. (Naturally, this DOES NOT apply to Apples’ Time Capsule.)

For some, Time Machine begins to perform as expected with a new external hard disk. But then the initial full backup or subsequent incremental backups fail. The user only later discovers the hard disk was still partitioned as Master Boot Record (MBR).

One article on Time Machine made this observation: “Virtually everybody will have to open Disk Utility and repartition the disk as APM or GUID. It doesn't really matter which one because the Time Machine disk will not be bootable anyway. APM allows a disk to boot a PowerPC, GUID allows the disk to boot an Intel processor but both are easily digestible by Time Machine on either kind of processor.” [http://www.girr.org/mac_stuff/backups.html]

Procedure
Connect the hard disk you wish to use for Time Machine backups.
Launch Disk Utility.
It will appear twice in the pane on the left. (Make sure you recognize that it is different from the 2 icons that represent your Macs' internal drive.) The upper entry represent the device as a whole, including the controller inside. The lower entry represents the hard disk contained within the device.
Click on the upper icon of the external hard disk.
Select the "Partition" tab.
For "Volume Scheme" choose "1 partition". (Chose 2 partitions if you intend on storing other data on the disk besides your Time Machine backups.)
Name the disk.
Format should be "Mac OS Extended (Journaled)".
Click "Options".
Select either GUID or Apple Partition Map. (See above for significance)
Click "OK".
Click "Apply". Then click “Partition”.

Once the external hard disk is repartitioned, select it again in Time Machine preferences and use it for your backups.

Let us know if this was helpful.

c14user,

With Disk Utility open, you see the drive listed on the left? It shows up as two icons right?

Click the upper one and select the "Partition" tab on the right. Type everything you see reported at the bottom of the window.

Cheers!