Time machine backup troubles

[mister-cotton]

Hi All. I'll try to be concise.

13" Macbook Pro early 2011.
OS = 10.9.5.

Trying to backup 1TB internal hardrive on 1TB external hardrive dedicated exclusively to Time Machine backups. Never had any trouble until recently.

On my internal, I'm only using 493 GB of 1TB. When I try to back up, the "Preparing Backup" takes forever and then it eventually says it failed. See photo below. I've deleted the "inprogress" file and there is currently only 1 backup on the external HD.

How are only 348Gb available on my external and why would it require 458Gb to successfully backup? Is this due to older files that may have been stored on my Mac at one time but no longer there? [At one point, the internal HD was almost full with media that's since been transferred to a stacked external HD for work].


Do I need to have an external HD that's larger than my internal HD to appropriately back up my computer?

Happy to provide more details if that helps. Thanks for any advice.

 

[mister-cotton]

Here is the screenshot.

 

[RMo]

Apple doesn't say anything specifically about this that I can find, but many online sources recommend a drive that is at least 2-3 times as large as the one you are backing up.

However, it's not the size of the drive your backing up that matters, really; it's the size of the data you're backing up. (So this shouldn't be a problem for you right now and shouldn't be much of a problem later as long as you don't mind not being able to go too far back in your backups.) If you did want to exclude some data from the backup, Time Machine automatically excludes some local cache folders, and you can add more to that list (in System Preferences) if you want, which could bring the size down. But that shouldn't be the problem here. As for old files that you've deleted, Time Machine will keep them (they are part of your backup after all) as long as it has space on the backup drive, but when that runs out it will start removing the older backups (and associated files) as necessary, and by default it will notify you as it does this.

It's hard to say what's causing this problem for you, but I'd guess that either you changed a lot of files since the last backup (unlikely if it's this many GB) or it doesn't recognize the disk as being associated with your computer's Time Machine, so it wants to keep the existing backup on that drive (which it doesn't care about) and create a new one, which you don't have enough room for. Or maybe, if you use the drive for other things, there could be items in the Trash taking up space, so you might try emptying that.

Personally, what I'd do is this: if you don't care about the backup that is currently on the drive, try reformatting the drive in Disk Utility (do an "Erase" and make sure it's formatted Mac OS Extended (or HFS+) with Journaling, which is probably the default). Then try setting Time Machine up with this disk again and see if it works.