I am having the same problem.  When trying to back up to TC it hangs on "preparing backup" for hours.

It didn't solve it for me:

"Appeared to work, but attempting to select Music from the Favorites resulted in loss of some historical windows, and they did not return with either repeated shift-command-c or attempting to select the hard drive in the Devices.

Too bad... I was really hoping!"

This didn't do it for me (shift-command-c)  Same problems.

I've lost all my backup history from april-2014. Even after reformatting the disk, TimeMachine under Yosemite behaves erratically, doesn't go back, doesn't respond to clics on tabs or buttons... In a totally new backup, with a history of two days.

Apple is becoming the worst software company in the industry in the late times. I simply can't figure out how this thing of Yosemite has been released. Really, beyond the dysfunctional design, where's the QA at Apple?

DO NOT UPGRADE. Stay on Mavericks.

You have to be logged in as an Administrator in order to access folders in your Time Machine. It took me hours to figure this out today.

I contacted Applecare about the Time Machine problem in Yosemite 10.10.1 on 11/18 (can't go back more than a couple of backups). Apple senior support rep did some screen sharing so I could demonstrate the problem and collected some log files from my iMac. The next day, I got a call back from Apple stating that they agreed that there is a problem with Time Machine for some users in Yosemite. The bad news us that there will be no fix until the next Yosemite update (10.10.2 ?). In the interim, you can manually restore files from the Time Machine using Finder.

Good suggestion Greenland. Have done and will continue. Also for all of you that have new iMacs you have free online assistance...I would worry them to death!! and be sure to tell them to read the comments on this site. Thanks again for the link.

Excellent job GroveMan! Thank you for the update. Fingers crossed for 10.10.2 !!!! (10.10.1 didn't work for me)

David Losada, whatever you do, do not erase your Time Machine disk! Time Machine is still doing its backup thing, accessing it is the problem. All of your backups are there, at least those back to the last automatic removal to make more space.

I'm the only user on my machines, and I'm the administrator, and I still get the Time Machine Backup Disk is Not Available error when attempting to access it via finder.

Re: John Gruver comment...By all means do not erase your Time Machine Backup File, reformat your drive, etc. because you will no longer be able to restore any historical save that you made in days, months, years ago will be lost forever. He is correct in saying that there are work arounds to getting this data back (re: Linc Davis comment) and in time, Apple will correct the issue. I have several large backup devices. If you have only one device and it is almost full, remember that Time Machine is erasing old and adding new, in this case you may want to add a new one or  turn time machine off  and save any important items the old way until 10.10.2 comes out a month or so from now. Just a thought.

Yes, it's true. TimeMachine has no problems in making the backup. It's the new interface that can't navigate properly the backup history.

But really, I can't understand the new Apple priorities. So much work in this terrible interface and they release the final version —even the .1 version—, with such an error in a critical component as is the backup system.

Quosque tandem, Apple, abutere patientia nostra?

I am using Time Machine on Mavericks, so this may not be relevant. However, both solutions worked so well, I thought I might share. There are tricks to make Time Machine work that may be applicable (NB This is only for Time Machine on NAS drives). If you get stuck forever at Preparing Backup, then mount the Time Machine so you get all the way to the .dmg. Mount that. If you see an xxx.inprocess, that is what is killing you. Use sudo rm -rf to nuke that thing. Sometimes it complains when you try that. You can also drag that .inprocess to the trash and empty. Both these operations will take a while. Note, one side effect is that Time Machine will never give the correct amount of space left on the drive again. Oh well.

One way to avoid Time Machine issues, is to try to avoid a situation where you start a backup, and then move the computer some place where it does not have wireless or ethernet access. You may want to turn Time Machine OFF when you are away from your network. Make sure that any backup is COMPLETED before you disconnect it. Or tell it to STOP the backup before you move. TM is supposed to know how to deal with this stuff, but I find it doesn't. Causing TM to start and stop leads to misery. The inprocess stuff sometimes fixes itself. Other times you have to force it.

Now, what about a nice big Time Machine (say, a 3 TB capsule) disk that has data that you still want to use but Time Machine has locked it, saying it is corrupt, and wants to erase it and make all new backups. Before you nuke it, or just put it away somewhere as an archive, note that, contrary to the myths, you can fsck a Time Machine volume. The instructions are here:

http://www.garth.org/archives/2011,08,27,169,fix-time-machine-sparsebundle-nas-b ased-backup-errors.html

For everyone's convenience, I have copied the contents of that website below. This worked PERFECTLY for me.

Fix Time Machine Sparsebundle NAS

Based Backup Errors

27.Aug.11

This is a modification of an original post for use when you have a corrupt sparsebundle backup on a NAS (as opposed to an external drive attached to a router) and it needs to be repaired. The NAS is likely a hardware product from the likes of Netgear, Synology, Buffalo or QNap – or for those of us with a home-grown backup server running FreeNAS.

The error you may see is “Time Machine completed a verification of your backups. To improve reliability, Time Machine must create a new backup for you.” This can be fixed by following the below.

From your Mac, connect to the network share that houses the sparsebundle.

At the top level of the drive are the various sparsebundles that make up your individual computer backups.

Do not double click on these sparsebundles or try to repair with Disk Utility.

Open Terminal and then switch to root by typing

sudo su -

and then enter your password.

The verication that has already run has marked your sparsebundle as bad, so first we need to make it look normal.

From the command line

chflags -R nouchg /Volumes/{name of your network share}/{name of}.sparsebundle

This may take a little while.

Now type

hdiutil attach -nomount -noverify -noautofsck /Volumes/{name of your network share/{name of}.sparsebundle

You will then see something like

/dev/diskx Apple_partition_scheme
/dev/diskxs1 Apple_partition_map
/dev/diskxs2 Apple_HFSX

Where x is the disk id for the external disk. You are interested in the one labeled Apple_HFSX or Apple_HFS. It might be 2, 3, 4 or higher.

At this point, I have found that the filesystem check is already happening. You can check for activity by tail’ing the fsck_hfs.log

NOTA BENE FROM MUNGOBOMB, I had to fsck manually SEVERAL TIMES.

tail -f /var/log/fsck_hfs.log

If fsck is going then in my experience it will be able to repair the sparsebundle. Go away for a few hours and let it chug away.

When it is done, you will either see

‘The Volume was repaired successfully’

or

‘The Volume could not be repaired’

If the latter you can run disk repair again:

fsck_hfs -drfy /dev/diskxs2

(Optionally if you have the available RAM, you can set a RAM cache in the command above to help speed up this command like so:

fsck_hfs -drfy -c 750 /dev/diskxs2

This will use 750MB of RAM – feel free to change this amount to best fit your system (amount of RAM vs size of your Time Machine Sparsebundle). If you are unsure about this, use the first command.

Make sure to replace x with whatever number your disk is from the output above.

The letters “drfy” tell the filecheck utility different things. d for ‘Show Debug’ – r for ‘Rebuild Catalog Tree’ – f for ‘Force’ and y for assume ‘yes’ to any prompts.

Now go do something for an hour or two. Come back and

tail -f /var/log/fsck_hfs.log

If all went well, the last output you will see is

‘The Volume was repaired successfully’

Now you need to type
hdiutil detach /dev/diskxs2

You can redo the above for any other Time Machine sparse bundles you have permission to modify while you have the network share attached to your computer.

Final step.

When complete, you need to edit an plist file within the sparsebundle that records the state of the backup. On the top level of the sparsebundle find a file called com.apple.TimeMachine.MachineID.plist. Edit it and remove these two nodes

<key>RecoveryBackupDeclinedDate</key>
<date>{whatever-the-date}</date>

Finally you want to change

<key>VerificationState</key>
<integer>2</integer>

to
<key>VerificationState</key>
<integer>0</integer>

Now you can eject the network share and have Time Machine give it another go. After the (long) verification step, backups should proceed once again.

Notes:

Ideally this should be done over a gigabit wired network connection. Do not attempt using Wi-Fi. You also want to make sure your machine does not go to sleep during the above operation.

[Update: 1.1.2013]

I appreciate all the warm feedback from people all over the world who have been helped by this post. This site helps to fund my hobbies, so if this post has helped you please consider a USD $1.99 donation to my hobby fund.

PLEASE PLEASE FUND THIS GUY. HE IS A LIFESAVER:

https://www.paypal.com/us/cgi-bin/webscr?cmd=_flow&SESSION=Ntxgb3gK8mBcMxzUXsKrC nUys3ex7h94Sbjp09qgodfLMpWjsmQ-OFIXY6e&dispatch=5885d80a13c0db1f8e263663d3faee8d 66f31424b43e9a70645c907a6cbd8fb4

Wow.  I know you're trying to help, but I am lost from line 1.  For the vast majority of Mac users, may I say that this is a job for Apple.

David: Absolutely spot on (including the Latin!).  You wonder how a company which seems pretty competent at writing software can make such an error and then take so long to fix it.

Nick.