I have done upgrades on our SRX 240 using ISSU multiple time but lately we are having issue upgrading from 12.3X48-D70.3 to 12.3X48-D75" WARNING: Not enabled dual root partition on secondary node ISSU not allowed" how do i over come this. Other option would be to upgrade using regular method but i want to stick with ISSU method for now.
ISSU is not supported on SRX240 so I'm quite surprised you have done it earlier.
Documentation states that SRX240 supports ICU which limits downtime to roughly 30 seconds.
More about ICU: https://www.juniper.net/documentation/en_US/junos/topics/task/operational/chassis-cluster-upgrading-and-aborting-backup-and-primary-device-with-icu.html
Branch devices only support ICU but the command used for ICU and ISSU is the same (request system software in-service-upgrade) and the confusion comes from there.
The problem reported is a known issue and you will need to move to D75 via the standard process:
I hope this info helps 😉
Were we able to fix you issue?