I have a working VM instance that I'm trying to copy to allow redundancy behind google load balancer.
A test run with a dummy instance worked fine, creating a new instance from a snapshot of a running one. 
Now, the real "original" instance have a persistent disk attached and this cause a problem in starting up the cloned instance because of the (obviously) missing persistent disk mount.
Logs from serial console output is as:

`* Stopping cold plug devices[74G[ OK ]
* Stopping log initial device creation[74G[ OK ]
* Starting enable remaining boot-time encrypted block devices[74G[ OK ]
The disk drive for /mnt/XXXX-log is not ready yet or not present.
keys:Continue to wait, or Press S to skip mounting or M for manual recovery`

As I understand there is no way to send any of this key strokes to the instance, is there any other way to overcome this issue? I know that I could unmount the disk before the snapshot, but the workflow I would like to instate is creating period snapshots of production servers, so un-mounting disks every time before performing it would require instance downtime (plus all the unnecessary risks of doing an action that would seem pointless).

Is there a way to boot this type of cloned instances successfully, and attach a new persistence disk afterwards?

Is this happening because the original persistent disk is in use, or the same problem would occur even if the original instance is offline (for example due to a failure in which case I would try to created a new instance from a snapshot)?