Hello
I use Ubuntu 18.04, and i want to know how to move correctly /home to another disk. I have two disk a ssd (OS) and a HDD (/home).
I noticed that i miss some GB’s on both, and i don’t know what is happening,
Thanks.
Welcome to the forvm.
It sounds like you have two unrelated issues here.
First, your home directory is designated by a mount point. You need to format the partition where you want the home directory to be to a supported fs (btrfs, extX, xfs, etc). Then you need to mount that device to a temp mount point to copy your home directories to the device. Then in your fstab, you need to set the device to be mount as /home. Then mount the device.
https://wiki.archlinux.org/index.php/Fstab
https://wiki.archlinux.org/index.php/Partitioning#/home
The missing GiB, you would need to clarify, but *nix systems properly use the binary metric system, so their actual reported size is what is really on the drive. Also, file systems have some overheads some some space will be used when formatting the disk.
MS Windows will give you two disk readings size and actual size on disk. Note how these sizes are never the same.
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To clarify; computers work with harddrives in terms of powers of two, while disk manufacturers work in the decimal system. This means the following table is used;
Unit | Manufacturer Computer
---------+-------------------------------------------------
kiloByte | 1 000 1 024
MegaByte | 1 000 000 1 048 576
GigaByte | 1 000 000 000 1 073 741 824
TeraByte | 1 000 000 000 000 1 099 511 627 776
PetaByte | 1 000 000 000 000 000 1 125 899 906 842 624So when you buy a 1TB drive, it’s actually a 909 GB drive according to how the computer reads it. Fun times! 
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Thank you for the clarification on that. It is much easier to see visually. I should have thought about that.
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