[j-nsp] Replacing RE in M7i

[Markus]

Hi,

a couple weeks ago the HDD in our RE failed and the box has since been
running off the internal flash (no PCMCIA flash installed). Last night
we replaced the complete RE with a new one but came across some
problems: since we originally wanted to keep our old JunOS version (7.4)
our Juniper partner advised us to simply put the current flash card into
the new RE and everything should work fine. We tried this, but it didn't
really work (internal flash was primary boot medium: /var (which JunOS
tried to mount from the HDD) could not be mounted on boot and thus many
daemons weren't started, only limited access to console and no CLI of
course). We ended up using the new RE as it was and loading our config
from a backup via FTP. Since there was a version jump (7.4 on the old RE
to the pre-installed 8.0 on the new RE) we were hesitant to do this, but
it worked fine, fortunately. 

So, my question is: what could we have done better in this particular
case if we wanted to keep downtime to a minimum? We're only in charge of
one M7i. Maybe simply replacing the broken HDD in the old RE and
issueing "request system snapshot"?

In particular I'm also wondering why the box didn't boot up fine after
we replaced the internal flash, and why /var could not be mounted. Maybe
it has something to do with 7.4 installed on the flash and 8.0 on the
new HDD? I'm a little lost here...

Thanks,
Markus