The disk can still be used, but there is no guarantee for how long it will be usable. If the disk is detached, it will be impossible to re-attach it later. In this case, do not detach the disk without a working CMK.
The disk can be detached and re-attached, and data on the disk will not be lost.
To ensure your data safety, you are advised to follow the instructions described in Disk Detachment Process.
Detach the disk from a running ECS. For details, see "Management" > "Detaching an EVS Disk from a Running ECS" in the Elastic Cloud Server User Guide.