By design, CMC has an Accidental Data Access Protection mechanism to prevent unintended data corruption to the migrated data.

Issue
While migration is in progress, the destination volume cannot be mounted.

Explanation
This behavior is by design. CMC has a Accidental Data Access Protection mechanism to prevent unintended data corruption to the migrated data.
During different phases of the migration process, CMC ensures that OS or application only have access to the correct copy of the data, as follows:

Initial Synchronization
Only source data is accessible by applications.

Periodic Re-Synchronization
Only source data is accessible by applications.

cMotion™
Source data is accessible by applications only via the migrated storage device.

At the cMotion™ state, both source and destination volumes are scrambled at-rest and are only accessible via the source block device in order to avoid data corruption. In rare cases such as catastrophic events where data has to accessed, follow steps below to manually descramble the the volumes.

Finalize Cutover
Source data is accessible by applications via the migrated storage device and destination data can be accessed directly.

Snapshots / Manual Descramble
However there are times where it is desirable to descramble a scrambled volume. For example, to make use of a snapshot clone of the destination volume which are scrambled if the snapshot was taken prior to Finalize Cutover, you will need to first manually descramble the data.

A separate utility is available for this purpose - manually descramble any block devices. After successful descrambling (within seconds), the volume can then be mounted without errors.

Warning! To avoid data corruption, DO NOT run this utility on any volumes that is currently in any migration sessions
Linux

[root@cds-centos75-c001 ~]# mtdi descramble /dev/sdd
Device /dev/sdd data will be modified [y/n]: y
device  /dev/sdd  descrambled successfully

Windows
Open cmd.exe or PowerShell as administrator and run the following command:
PS C:\Program Files\CirrusData\mtdi> .\mtdi.exe descramble \\.\PhysicalDrive1
Device \\.\PhysicalDrive1 data will be modified [y/n]: y
device  \\.\PhysicalDrive1  descrambled successfully
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