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I have a 1TB harddisk that has 20GB space left. I want to encrypt this disk. Will encryption take up more space, and if so, is there an estimate, a percentage or something how much it will cost?

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When I open Diskutility, I see a change for partitions that are encrypted. They each have two entries, a parent and child. The parent is the normal partition, and inside is the encrypted child partition. As the encryption is always done by the OS, it is software encryption.

The resulting encrypted child partition is smaller than the parent. As I had several encrypted partitions, I was able to compare them using the numbers in Diskutility. I can only give you the numbers from this one example, so I don't know if they are similar on other systems. For this computer, encryption takes 320MB data. One partition (root) uses 4KB less for encryption. Disk Utility reports for the parent root partition that 4096 Bytes are free. The other parent partitions have 0 bytes free. Why this is - I have no idea.

Below you see the data in bytes for all three partitions: 150GB, 300GB, 850GB.

(A)Partition   (B) Parent      (C) Child      (D) Difference  (E) MB
1 (root)       149484212224    149148663808   335548416       320.00390625
2 (external)   300060016640    299724464128   335552512       320.0078125
3              849726693376    849391140864   335552512       320.0078125

Difference D2 - D1

335552512 - 335548416 = 4096 = 4KB

As I use 256 bit encryption, these numbers may be different for 128 bit. If anyone can confirm this, that would be nice!

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  • So it takes more, but not enough to worry about. Nice work. i do wonder if it needs more to do the initial encryption, space to work in etc
    – Tetsujin
    Oct 19 '14 at 13:18
  • On Yosemite the difference is of 371 MBytes.
    – dan
    Oct 5 '16 at 10:20
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The chances are you don't have enough free space on it to allow it to encrypt at all; if it does work it will be horrendously slow.

You should always leave at least 10% free space on a hard drive, preferably 15%

Encryption is not by default also compression; the resulting file size may be no different.
For compression, if the data is predominantly 'text' then compression will save space; if it is music, pictures or video it will save none.

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  • The OP isn't asking about a separate encrypted .dmg on a HD plenty large enough to take it. An Encrypted .dmg is simply the size you set it to. If it's only got 18MB of data on it, you can shrink it to 20MB & it will still work just fine. Totally different concept.
    – Tetsujin
    Oct 18 '14 at 10:22
  • I use Superduper to copy all content to an external harddisk. Then I encrypt the disk, then I copy it back. I know I should clean up or buy a 2TB disk, but this is as it is right now, and I'm just curious. I'm not asking about an encrypted DMG. I'm going to encrypt the entire partition.
    – SPRBRN
    Oct 18 '14 at 10:26
  • This is not the system partition. OSX is installed on another partition, so it won't affect system cache or something.
    – SPRBRN
    Oct 18 '14 at 10:29
  • Without knowing the precise details; encryption means adding some data, a hash etc at minimum, so unless there is also some compression being done at the same time [with the same caveats as above] then the result will likely be larger than the original.
    – Tetsujin
    Oct 18 '14 at 10:29
  • or, another option - try it & see. As it's a duped drive anyway, you're risking nothing. All I'd beware in addition is that, afaik, there is no progress indicator other than the disk's read/write light.
    – Tetsujin
    Oct 18 '14 at 10:31

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