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I was considering using Time Machine and I was wondering if creating a 50 GB partition on a disk of 500 GB for it make sense.

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Using the same disk for system and time machine makes sense only if you want to be able to retrieve deleted files on a laptop. Combining this with an external backup of both partitions would be wise. – mouviciel Oct 14 '10 at 9:10
@mouviciel That's exactly what I have explained in the comments below. I should have added to the question. – Patrick Oct 14 '10 at 11:31
up vote 4 down vote accepted

Technically you could do this, although 50 GB is far too small for most any use these days.

The point of concern I think should be raised here is that you look to be hosting both files and backups on the same drive. When (not if—when) the hard drive fails, you will lose both. You would be much better off in the long run keeping the files and backups on separate drives.

UPDATE (based on comments): The rule of thumb that I've seen for deciding on a drive size to house a Time Machine back up is at least a 2:1 ratio of the drive to be backed up (so a 500GB would mean a 1 TB TM-hosting drive), and I don't see why that general rule couldn't be followed here.

Look at the data volume of the folders you intended to back up, double that, and that's the size partition you go with. If Time Machine requires a minimum that's below what you calculated, just double the size of that for your partition and off you go.

Either way, I don't think you're going to find a hard number here.

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@Philip Regan yeah.. that's true and a good point.. thanks. I have a backup for that (I do it manually...). However I wanted to use time machine just to recover files (such as when I erroneously save some text file and I lose code.. etc). What do you think a good ratio should be ? (500GB hard disk) – Patrick Oct 13 '10 at 20:29
If you already have another drive for hosting files, then why partition this new drive? Just leave it as is. – Philip Regan Oct 13 '10 at 20:44
@Philip Regan: Because I need automatic backups on short-time period mistakes.. such as overwriting wrong files etc. In other words, I want an immediate backup, (every time I save a file) – Patrick Oct 14 '10 at 6:13
Right. I get that part, I use TM at home. What I think is confusing a lot of people is why you feel the need to partition the disk. Because based on your comments, you don't. – Philip Regan Oct 14 '10 at 9:51
@Philip Regan OK, so.. well let's say I exclude all folders and I decide to backup only few of them (mainly text files).. how big should the partition be ? What about 50GB ? – Patrick Oct 14 '10 at 11:32

You're interested in using the 50GB partition for Time Machine? Keep in mind that Time Machine keeps multiple copies of changed files - it's not just a single back up of your drive. Your backup drive should be at least a couple times larger than the stuff you're backing up, so Time Machine can keep a decent backlog of changes beyond just the most recent.

That said, Time Machine will fill the disk with backlog changes until it can no longer fit them, so as long as you have enough space for a "most recent" backup, Time Machine will be happy.

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@Nick ok thanks, so let's say I have a 500GB hard disk (and I don't use big files.. just work and programming..) what a good ratio for a partition ? what about 300GB for my files and 200GB for time machine ? – Patrick Oct 13 '10 at 19:21
That depends entirely on how big your "work and programming" files are. Maybe you work in Final Cut Pro (big files) or just textedit (small files)? – Nick Oct 13 '10 at 19:27
@Nick ok. No.. just web and mobile development (no big files). Is not possible to decide which folders to backup right ? It would be great – Patrick Oct 13 '10 at 20:30
You can choose to not back up certain folders, which could be most of your drive if you choose. – Nick Oct 13 '10 at 20:41
@Nick ok cool, so then.. this change everything, because If I only backup text files folders.. I just need few GB for the back up right ? – Patrick Oct 14 '10 at 6:08

TimeMachine already places backups inside a folder on the hard disk, so it will not create a mess on that drive.

The only reason you should create a partition is if you would not like TimeMachine to use up as much disk space as it needs. Every time a file changes it will create a new copy of it in the backup, and only pruning old versions once the disk is full. If you create a small partition it will restrict the backup to that size.

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