Ok
I've just tested the second solution I've given you.
vmtest (112) is my healthy VM:
root@pve1:/etc/pve/nodes/pve1/qemu-server# cat 112.conf
#Debian 8
#
#Test VM ...
bootdisk: ide0
cores: 1
cpuunits: 512
ide0: drbd1:vm-112-disk-1,cache=writethrough,size=32G
ide2: none,media=cdrom
memory: 512
name: vmtest
net0: e1000=82:04:F9:0B:63:BD,bridge=vmbr0
numa: 0
ostype: l26
smbios1: uuid=2b6c9ce9-0ad5-4e47-a525-244051a6919b
sockets: 1
vmsupp (101) is my broken VM:
root@pve2:/etc/pve/nodes/pve2/qemu-server# cat 101.conf
#Debian 8
#
#Former x2go VM (to be decom)
balloon: 512
bootdisk: ide0
cores: 2
cpuunits: 2048
ide0: ds412:101/vm-101-disk-1.qcow2,size=32G
ide2: none,media=cdrom
memory: 2048
name: vmsupp
net0: bridge=vmbr0,virtio=62:65:38:63:66:62
numa: 0
ostype: l26
smbios1: uuid=7088cbe6-6c8a-48c2-b874-792995416a32
sockets: 1
I've added (vi) vmsupp's disk to vmtest as ide1:
ide0: drbd1:vm-112-disk-1,cache=writethrough,size=32G
ide1: ds412:101/vm-101-disk-1.qcow2,size=32G
Then start vmtest (it needs to be stop/shutdown and start, a simple reboot is not enough):
root@vmtest:~# fdisk -l | grep /dev
Disk /dev/sdb: 32 GiB, 34359738368 bytes, 67108864 sectors
/dev/sdb1 4096 2152447 2148352 1G 83 Linux
/dev/sdb2 2154496 67108863 64954368 31G 83 Linux
Disk /dev/sda: 32 GiB, 34359738368 bytes, 67108864 sectors
/dev/sda1 * 2048 64286719 64284672 30.7G 83 Linux
/dev/sda2 64288766 67106815 2818050 1.4G 5 Extended
/dev/sda5 64288768 67106815 2818048 1.4G 82 Linux swap / Solaris
root@vmtest:~# mount /dev/sdb2 /mnt
root@vmtest:~# cat /mnt/etc/hostname
vmsupp
Done.