I would just 'request virtual-chassis routing-engine master switch' then uncable it from the VC. Do a format install from the boot loader, and once it's back up recable to the VC.
USB: To perform this recovery installation, the USB Drive should be formatted to FAT-32 & should be empty. Copy the Junos package to USB flash drive. Power off the EX switch Put the USB flash drive in the EX switch. Power on the EX switch. After the 'Loading getting /boot/default/loader.conf' displays, hit space bar for the loader> prompt. Issue the install command with the format option: loader> install --format file:///<Junos package name> For example: loader> install --format file:///jinstall-ex-4200-10.4R2.6-domestic-signed.tgz