Thanks to your advice, JerryF, I tried BackinTime and, as it were backupping, I realised that, as I asked it to save the / partition, it was also saving an other whole hard drive witch I'm using to store docs (NTFS)... So I stopped it and looked a little closer and I discovered a /boot/efi partition on that hard drive... (Incidentally, I figured I also can't see that HD in the file explorer of LMDE...)
Is it why it's trying to save that disk to ?
That could explain why there was not enough place for timeshift...
I wonder why this /boot/efi partition is on that HD... It's not even the HD where windows 7 is installed...
Maybe I will have to settle that question of /boot/efi partition first... and re-install LMDE properly ?
These are screenshots of Gparted showing :
The HD where I installed LMDE :
The HD where Windows 7 is installed :
The HD where I found that /boot/efi partition and that BackInTime is trying to save :