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I am trying to encrypt an external drive using diskutil CoreStorage on Mac Lion 10.7.4. I thought the only requirements were that the drive have GUID partition scheme and Journaled HFS+ file system. I think my drive is configured accordingly but when I type the following command I get an error message back:

Michaels-MacBook-Pro:~ Michael$ diskutil cs convert disk2 -passphrase TestPassword

Error converting disk to CoreStorage: The given file system is not supported on Core Storage (-69756)

Here are the details reported for the drive in question:

Michaels-MacBook-Pro:~ Michael$ diskutil list disk2
/dev/disk2
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *500.1 GB   disk2
   1:                        EFI                         209.7 MB   disk2s1
   2:                  Apple_HFS Test1                   499.8 GB   disk2s2

Michaels-MacBook-Pro:~ Michael$ diskutil list disk2
/dev/disk2
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *500.1 GB   disk2
   1:                        EFI                         209.7 MB   disk2s1
   2:                  Apple_HFS Test1                   499.8 GB   disk2s2

Michaels-MacBook-Pro:~ Michael$ diskutil info disk2s2
   Device Identifier:        disk2s2
   Device Node:              /dev/disk2s2
   Part of Whole:            disk2
   Device / Media Name:      Test1

   Volume Name:              Test1
   Escaped with Unicode:     Test1

   Mounted:                  Yes
   Mount Point:              /Volumes/Test1
   Escaped with Unicode:     /Volumes/Test1

   File System Personality:  Journaled HFS+
   Type (Bundle):            hfs
   Name (User Visible):      Mac OS Extended (Journaled)
   Journal:                  Journal size 40960 KB at offset 0xe8e000
   Owners:                   Disabled

   Partition Type:           Apple_HFS
   OS Can Be Installed:      Yes
   Media Type:               Generic
   Protocol:                 FireWire
   SMART Status:             Not Supported
   Volume UUID:              1024D0B8-1C45-3057-B040-AE5C3841DABF

   Total Size:               499.8 GB (499763888128 Bytes) (exactly 976101344 512-Byte-Blocks)
   Volume Free Space:        499.3 GB (499315826688 Bytes) (exactly 975226224 512-Byte-Blocks)
   Device Block Size:        512 Bytes

   Read-Only Media:          No
   Read-Only Volume:         No
   Ejectable:                Yes

   Whole:                    No
   Internal:                 No

I'm a little concerned that the "Partition Type: Apple_HFS" entry is causing the problem, but I don't know how to change that. I only seem to be able to control the "File System Personality: Journaled HFS+" in Disk Utility.

Can anyone shed some light on this for me?

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You need to give it your JHFS+ partition (disk2s2), not the whole disk (disk2).

The man page for diskutil, when covering the coreStorage convert (cs convert) verb, specifically says this:

Convert a regular Journaled HFS+ or Case-sensitive Journaled HFS+ volume (must be on a partition and within a GPT partitioning scheme) into a CoreStorage logical volume.

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