The Backup Disk is a quite useful item to keep in your inventory. Think of it as a life insurance: this rare passive powerup will automatically trigger on the first incoming blow that would otherwise prove to be lethal. Instead of dying, the disk will fill back half your lifebar. You can carry around one disk in both inventory slots at any time. Reassuring for sure, but any disks you keep with you might take up space from other useful items, so the choice is up to you: would you rather stock up on safety nets, or give room for other items you might encounter on you run?