To repair a disk using the unix commands in Terminal.

To repair a disk it should not be the boot disk, and you must have root privileges.

Enter:

#diskutil verifyDisk /dev/disk*

* to find out out the disk number you can enter: #diskutil list

(for example : #diskutil verifyDisk /dev/disk0s3 )

Now, verifyDisk will do its small tests, if there is a problem, type
#diskutil repairDisk /dev/disk*

Attention diskutil function can do a lot of things on your disk, this means also formatting and deleting your complete disk. For a list of options type
#man diskutil (man lists the option of a function)

Well, last thing useful for small repairs (after checking) :

#diskutil verifyPermissions /dev/disk*

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