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What do I do when my backup drive fills up?

I have what I thought was a fairly big (230GB) extra hard drive fitted to my
PC which I use for backup purposes. The drive is slowly filling up with all
these Incremental backups (58GB free space) left! Can I get rid of any of these
old backups? If so, how do I choose?

No matter what tool you use, a properly configured backup is likely to
simply collect more and more data in the form of backups as time goes on. If
you’re backing up to an external drive eventually it will fill up, no matter
how big it is.

What to do, meaning what to remove, depends on the type of backup you’re
doing, what you expect to be able to use your backups for and what other
storage options you might want to use.


In an earlier article, I discussed the differences between “full” and “incremental” backups. In short, a full backup is a complete backup of absolutely everything on your machine at the time the backup was taken. An incremental backup includes only those things that have changed since the immediately preceding backup was performed.

“If you’re taking incremental backups, as I suspect most do, things get a bit trickier.”

If you take full backups each time, then while your disk will fill up much quicker, your options are actually fairly simple: just pick which ones you want to delete. Each backup stands on its own, so there’s nothing to worry about. Perhaps keep yesterday’s, last week’s and last month’s, and delete everything else. (More on the logic behind that seemingly random example below.)

If you’re taking incremental backups, as I suspect most do, things get a bit trickier.

An incremental backup is typically useless on its own. It relies on the incremental backups that came before it, all the way to the initial full backup, to create a picture of the data being backed up. Thus, when you want to clean up, you need to make sure to retain all those.

The best way to think of it is that the initial full backup, and all the incrementals that come after it, comprise one backup set. You can delete it, or not, but only as a complete set.

Now if all you’ve been doing is incremental backups since day one, then you can’t really, safely, delete anything. Removing the initial full backup, or any of the incrementals along the way would cause all the backups in the set taken thereafter to be invalid.

That’s why I recommend a blended approach. Take periodic full backups, and daily incrementals. That way each time you take a full backup you “reset the clock”. That full backup stands on its own, and any of the preceding backups, incremental or otherwise, can be safely deleted depending on your own needs and plans.

Many backup programs, including my recommended Acronis Trueimage, can now actually automate this periodic full and daily incremental approach.

And, indeed, that’s my approach: once a month I take a full backup of my machine, and every night an incremental. Over the course of a month that pretty much fills up the external drive I happen to use, so each month I start over.

Once we know what’s safe to delete from a technical point of view – anything prior to the most recent full backup would be a good rule of thumb – we need to think about what it is we need our backups for.

The most common need for a backup is to recover from a system crash. That is, you’re going along, your system dies, and you end up using the most recent backup to restore your system to its prior state. For me, that means if my machine dies, I can restore it to whatever state it was in as of the last incremental backup taken in the middle of the preceding night.

If that’s all you expect to need, or the only case you need to care about, then that rule of thumb not only defines what’s safe to delete – anything prior to the most recent full backup – but the minimum of what you need to keep – the most recent full backup and all subsequent incrementals. That will always allow you to restore as needed to the most recent backup in case of a catastrophe.

But backups can be useful for more than that.

It’s not terribly uncommon to want something older than the most recent. Perhaps you installed a virus or some spyware and didn’t realize it for a few days. Perhaps you want to recover a document that you deleted last month. Perhaps you’d like to restore your machine to the relatively pristine state it was in shortly after you received it.

All of these scenarios, and more, can be accomodated by keeping the appropriate backups. You don’t have to keep them all, just a select few.

Using myself as an example, again, I:

  • Keep all the monthly full backups for roughly three months.

  • Keep the quarterly backups (the monthly full backup from January 1st, April 1st, July 1st and October 1st) for at least six months, if not a full year.

  • Keep the yearly backup (that January 1 full backup) for as long as I can.

  • If the machine did not come with installation media (or often, even if it did) I keep the first full backup for as long as I have the machine.

If my math is right that’s a storage requirement of about 6-10 times the size of a full backup throughout the year, growing by one each year as I keep a relatively permanent archive.

Do I need to keep that many? Probably not, but disk space to store it on is relatively cheap.

Note that I keep only full backups. As I said above, they stand on their own; I don’t need to keep track of associated incremental backups at all past the current month’s daily backups.

So what you actually need to keep really depends on your own needs and plans. At a minimum, the most recent of course. But if you can envison using backups for more than just restoring to yesterday’s machine or grabing a file you just deleted by accident – you might consider setting up a system that allows you to keep a few of those snapshots as you move forward.

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8 comments on “What do I do when my backup drive fills up?”

  1. Hi
    I use an external backup system -a 500 GB MY BOOK-that I am happy with using. My question is: should it be turned OFF when I am not just backing up my information? I backup my photos/documents and music files. Is this system at risk of a virus/spam/malware etc if I leave it on just as the C drive is?


    There is a risk, and many people will tell you to turn it off when not in use. Some malware will cross to any drive it can find on your machine. That being said … I leave mine on all the time.


  2. Incremental backups normally work by backing up files for which the archive attribute has been set. Unfortunately, this attribute is set by many programs when they open a file, even if they don’t change it. Also, programs (including the operating system) often make ‘housekeeping’ changes that you don’t necessarily need to keep. This can greatly increase the size of your incremental backups.

    I try to back up only files that I intentionally change. It’s quite difficult to do this, so I won’t go into details, except to say that I find the program xxcopy, which is a much more powerful alternative to xcopy, very useful in this regard. As a result, my daily incremental backups are usually only a few megabytes in size, whereas Leo says, in another article, that his are a few gigabytes. (I don’t doubt that he works harder than I do, but probably not that much harder :-).)

    Periodically, I copy my incremental backups from the hard disk to DVDs, before deleting them from the hard disk, but retaining indexes to them on the hard disk. This way, I can get back any version of a file I’ve worked on (except, of course, ones that were overwritten during a single day’s work).

  3. Re. “What do I do when my backup drive fills up?” I’m using Vista Business, and do daily backups to an external hard drive (we’ll call that BU Drive #1) using the Microsoft BackUp program built in to Vista. I have a second external hard drive which I also back up to (we’ll call that BU Drive #2) , and that one is kept off-site ie when I leave the building it comes too. Back Up Drive #1 is rapidly filling up. I would have thought that I could just delete all backups on it and then start again with a full-system backup, but it appears that the operating system on the computer keeps track of all the backups done previously and assumes that they still exist on the backup drive. Is this the case? Can I just wipe the backup drive and start afresh with a new complete backup, or must I delete the old backup log files on the computer too? If so, where would I find the log files? Thanks, Steve Hunter_Melbourne Australia

  4. yes, I have the same problem what to delete safely, I have two external hard drives for two differet computers, if I want to restore will it be a full restore or only partial…I just use back up on windows is that a full or partial one thank you

  5. I am undertaking a full backup of my computer onto an external hard drive, i also have backups on my computer,I’m using the Windows XP platform.My question is: Is there any point to retaining my on-board computer backups and will I gain significant hard-drive space if i can delete my on board computer backups?

  6. Leo,
    My internal disk (750GB) is about to become full due to a tremendous amount of video from my camcorder. I’m thinking of moving all my video to an external drive to free up my internal drive for other things. If I do this, what is the best to backup that external drive?


    In short: another external drive, and backup software to automate the process.

  7. Great Leo! The first clear (jargonless) strategy I’ve heard, on incremental & full backup, even from the manufacturer’s forum. Thanks a GB!

  8. Will Macrium delete an old backup when the backup drive gets full? If not, is there any software that will automatically remove old backups from the backup drive when it gets full?

    There are in fact options in Macrium to do exactly that.


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