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Hello yihua,
The “unmount” process is done at the OS level while the detach process is an AWS operation. If the EBS is just an additional volume, then you must unmount it first and detach. You can’t unmount a root volume. In the question, the instance is said to be an EBS-backed instance which implies that it is using an EBS volume as its root volume.
If an EBS volume is the root device of an instance, you must stop the instance before you can detach the volume.
Let me know if this answers your query.
Regards,
Carlo @ Tutorials Dojo
