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How large is large enough when it comes to back up storage media

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As recommended to me by Bobba_3C0X1, I am following the 3-2-1 IT backup method.  I have completed my online cloud system image and now I am looking to set up my local backup.  My online system image backup clocked in at right around 800 GB.    I found a good deal on a 4tb USB 4.0 Ext. Hard Drive and am wondering if the 4 TB will allow sufficient room to perform a differential back up scheme.  

 

Dean.

 

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Dean, I would have thought that a 4TB external hard drive should be more than adequate for your Entire PC backup assuming that you are not adding large amounts of new data that will cause any differential backups to be very large in size.

The other point to note is that you should also avoid doing a full defrag of your source drive(s) as that can also cause any subsequent incremental or differential backups to be significantly larger than they would be otherwise, this is because defrag touches a very large number of sectors and therefore is detected as changes to the source data.

Until you have made your first differential backup, based on your normal system activity and data changes, then you won't really know how large each differential image might be and how many you can store along with further full backup images.

Dean,

Of course it depends on your backup scheme. Whether differential or incremental, it is recommended to set up the scheme so that you have new full backup from time to time.

My recommendation for the "from time to time" parameter is the following: you should not let your last full backup grow so old that you would not practically restore that backup, either because of system updates, software installation, or because of your personal content.

If personal content is the most demanding criteria, consider doing a different backup for content. You can supplement your Acronis backup with file backups. I recommend file backup technology that keep the original file format, and keep files individually copied "flat" (like Windows File History, GoodSync, etc).

Now that you have determined the length of your backup chains (full + partials) measure the average size of the chain or estimated it based on a couple of runs.

Next, you need to determine how long you want to retain these chains. The recommended setting is "keep n most recent chains".

If the number of chains is n, the size of the chains is YGB, and the size of a full backup is X GB), then the size of the backup destination should be: (n*Y+X)*(1+30%)

The +X is coming from the fact that ATI will first do a new full backup before deleting any older chain. The 30% is a margin for safety to account to the variability in the backup size.