Just to be safe i decided to do a backup with TWRP after the installation of CyanogenMod. Previously i did a backup of the stock rom.
Now if i TWRP -> Restore I'm able to see only the last backup done, and if i use a file browser i'm only able to see the old stock backup.
What is happening? I wish i could delete the old backup to get back the 2.1 GB it occupies
Edit:The issue is not related to the device, but to the different android version of the rom. Previously my backups were in /TWRP/BACKUP, now in mnt/shell/emulated/TWRP/BACKUP