Describe the bug
On Alpine Linux, for instance, the Disk entry is left undetected.
See d676b0b#diff-e6076e0f8c9ab0586c8909ec8d6832beR67.

Additional context
When fixing this (let's say with a simpler df -P -B M call and a proper columns parsing), we've to think about an optimized approach, as while checking for BTRFS mount-points, the second df call would also fail...
Maybe this fix could be thought over while reworking Disk for #55.