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?
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!
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.