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Re: System won't boot anymore after upgrade to jessie

looks like initrd and/or udev
There were similar issues with upgrade from wheezy to jessie in my upgrades
as well.
I usually boot with usb stick and fix the initrd - you might need to
configure or reconfigure few things. the easiest to try is to recreate

Yes, all the hints point to that, but I'm not sure I can do this alone without making a mess and with a machine containing several Terabytes of data I'm not allowed to :_) 
But as you did not provide any information how system is setup regarding
lvm, its just guessing

It wasn't me to configure the machine in the first place so is a bit tricky. I can give more information if you can point me out what you could need. thank you a lot