Q: How do I undo "Remove Disk" from Time Machine?

I don't think that you did what you think you did. There is no such functionality, AFAICT.

You probably added or removed this disk from the list of things NOT to backup, not from the list of backup disk options...

Just go to System Preferences->Time Machine, click the Select Disk... button. If your drive is connected to your mac and is formatted as Mac OS Extended (Journaled), it must appear.

Roanoke, I had the same experience with a My Book Live, and I think it happens when you remove an MBL from Time Machine's "Backup Disks" list at a moment when Time Machine isn't recognizing the MBL as an available backup drive for some reason.  In this case, Time Machine not only deletes the MBL from its "Backup Disks" list as you requested, but it goes a step further and deletes it from its "Available Disks" list, too.  This is a surprising, unintended side effect of the operation you tried to perform, and it might leave you with the impression that you committed some kind of dreadful, irreversible user interface error, but you didn't.  It would have been helpful if Time Machine had told you why it was performing the extra, unrequested deletion.

There are various reasons why Time Machine might stop recognizing an MBL as a backup drive.  In my case, the problem was related to a bad router I was using temporarily.  This router didn't properly support Bonjour, which is what Time Machine uses to find network backup drives.  To fix the problem, I had to replace the router, then I shut down the MBL from its web interface, unplugged the unit, let it sit for a few minutes, and plugged it back in to reboot it.  When it came back up, Time Machine once again recognized it as an available backup drive.

It's also possible that Time Machine isn't recognizing your MBL as an available backup drive because of some configuration change you made on the MBL, if indeed you did anything like that, or because your router needs a firmware update to support Bonjour.

Most of my remarks about the MBL apply to Network-Attached Storage (NAS) drives in general.  Luis is correct in saying there's no way to explicitly remove a drive from Time Machine's "Available Drives" list, but a NAS drive is a different animal from the plug-in type of drive he's talking about.  In particular, a NAS drive doesn't need to be in HFS+ format to support Time Machine, as long as it "speaks" Apple File Protocol (AFP) fluently.

Hope this helps.

I too have made this mistake and have tried all the things suggested on this thread with no luck - did you find any solution?

I have gone through the list of options with Apple tech support and am now at the point of trying erasing everything on the macbook pro and reinstalling the operating syste. I really don't want to have to try this unless it is absolutely necessary.

SUCCESS!!!

It seems that removing the disk in time machine may have actually disabled the Time Machine backup option in the MyBook Live software. In the browser interface I went to settings>system>backup settings and checked the box for Time Machine backups. It immediately reappeared in the Time Machine select disk dialogue.

Roanoke, I think your question was directed at cirqular, but I'll take a shot at it.

The "browser interface" is the way you control your My Book Live through its built-in Web server.

From the browser on your Mac, surf to this address.

     http://mybooklive.local.

(The period at the end is intentional.)  This should take you to your My Book Live's home page.  If it doesn't work, your router may not be supporting Bonjour properly, which could be your problem.

Once on the home page, click on Settings, click on System (if it's not already selected), click on Mac Backups, and make sure the Enable box for Time Machine Backups is checked.  The Select A Share box should be set to TimeMachineBackup by default, and the Maximum Size for backups should be set to the size of your drive, or a reasonable fraction of that.

Let me know how you get along.

I have a simlar issue. I have been backing up to an external HD plugged into another Mac for years (shared drive). Long story, but I instructed the Time Machine to remove the drive from available backup drives. BIG MISTAKE. I can not undo this. I have deleted the plist file several times. What is interesting is that if I change the name of the shared drive, it immediatley pops on the list of available backup drives. I would prefer to NOT change the name because another Time Machine backs up to this same external/shared drive. I have also failed many times at trying to use terminal commands to have the TM inherit the sparsbundle. Note that I can mount the sparsebunlde, and I  see that all the data is in there. TM just refuses to show this shared drive as an available backup drive. I feel there must be some other plist file that tells the TM not use list the drive, or there is something written to the drive itself that is marking it to exclude it from the list.  FYI: I use an Airport Extreme as a router. Screen sharing and sharing drives/folders across the network is set up and functions properly.

Found the Answer!

Works for encrypted Time Machine backups too!

OS Type: Mavericks

How to undo undo remove disk in time machine:

1.  Connect your external Time machine Hard disk, which currently does not appear in Time machine under "Select Disk", to your mac.  Have time machine running in the back.

2.  From your desktop menu bar: Go > Utilities > Disk Utilities

3.  In Disk Utilities, on the left panel, you will see your external time machine disk. Click on it and press the big blue mount button. This should prompt Time Machine to ask you whether you want to use this device for backups. Press yes and that should do it.

4. If you encrypted your backup, you will see your external time machine hard disk name appear twice. Double Click on the second one (it will be tabbed below the first one) and it will ask you to enter in your encryption password. Once you enter in the password, Time Machine will ask you whether you want to use this device for backups. Press yes and that should do it.

Spent 4 hours on trying to figure this thing out and apple did not have this in their documentation.

Hope this helps!

Hi kingsun3 - i got to through the stages you suggest but my external HD won't mount. Any tips on why this is? I made the mistake of removing this hard drive as my Time Machine and now it doesn't show up like it happened to others in the thread. Thank you!

Open Activity Monitor in Applications>Utilities, select All Processes & see if fsck is running, if so highlight it & force quit it.

See mount section in 1st linked article.

Disk Unmount Using Terminal

Disk Utility – Force Mount Disk