How can I encrypt a disk where diskutil cs convert reports “a problem”, leaves disk unreadable?

I'm trying to encrypt my Time Machine backup volume, but diskutil refuses, and leaves the disk unreadable (but repairable using Linux). How can I proceed?

It's a Western Digital Elements 107C 3TB USB3 disk.

It has a single HFS+ partition and has been working fine.

diskutil verifyDisk and diskutil verifyVolume report no problems. I have also used Alsoft DiskWarrior to repair discrepancies, without problems.

If I try to convert the volume, diskutil report "a problem", like this:

rb@Silverbird$ /usr/sbin/diskutil cs convert /Volumes/RB3TB1/ Started CoreStorage operation on disk4s2 RB3TB1 Resizing disk to fit Core Storage headers Creating Core Storage Logical Volume Group Attempting to unmount disk4s2 Switching disk4s2 to Core Storage Waiting for Logical Volume to appear Mounting Logical Volume A problem occurred; undoing all changes Switching partition from Core Storage type to original type Undoing creation of Logical Volume Group Reclaiming space formerly used by Core Storage metadata Error: -69842: Couldn't mount disk

This leaves the disk unreadable. diskutil cs list shows the disk as a Core Storage volume which is "online" and "revertible" but it cannot be mounted or acted on, neither can the parent physical volume. Disk Utility hangs on launch when the disk is connected. diskutil repairDisk on the parent volume reports:

Repairing the partition map might erase disk4s1, proceed? (y/N) y Started partition map repair on disk4 Checking prerequisites Problems were encountered during repair of the partition map Error: -69808: Some information was unavailable during an internal lookup

In fact, all access to the disk appears to be broken at this point.

rb@Silverbird$ sudo /usr/sbin/gpt show /dev/disk4 gpt show: unable to open device '/dev/disk4': Input/output error

You can't even read the device using hexdump. The kernel seems mightily confused.

I was able to recover the disk by mounting it in Linux and using the "gdisk" utility to change the partition type from AF02 (Core Storage) to AF00 (HFS+). After that, diskutil verifies both the disk and the volume as OK.

I suspect Apple has a bug where diskutil fails to revert the partition type.

But this gets me no closer to encrypting my backup volume.

Does anyone have any idea what the "problem" might be and how I might proceed?

rb@Silverbird$ /usr/sbin/diskutil info /Volumes/RB3TB1 Device Identifier: disk5s2 Device Node: /dev/disk5s2 Whole: No Part of Whole: disk5 Device / Media Name: RB3TB1 Volume Name: RB3TB1 Mounted: Yes Mount Point: /Volumes/RB3TB1 File System Personality: Journaled HFS+ Type (Bundle): hfs Name (User Visible): Mac OS Extended (Journaled) Journal: Journal size 229376 KB at offset 0x8f07408000 Owners: Enabled Partition Type: Apple_HFS OS Can Be Installed: Yes Media Type: Generic Protocol: USB SMART Status: Not Supported Volume UUID: F096E831-F27D-3433-9BBE-6B65F4F69FA5 Disk / Partition UUID: FE09034E-6AA0-4490-82A1-1F7E894ACD91 Total Size: 3.0 TB (3000110108672 Bytes) (exactly 5859590056 512-Byte-Units) Volume Free Space: 78.1 GB (78142939136 Bytes) (exactly 152622928 512-Byte-Units) Device Block Size: 4096 Bytes Allocation Block Size: 4096 Bytes Read-Only Media: No Read-Only Volume: No Device Location: External Removable Media: No

The disk contains around 732000000 files, according to Disk Utility (actually directory entries).

This is OS X 10.11.2, all up to date, based on a clean install of OS X 10.11 last month.