mount $BLK_DEV /mnt
Find the Debian mirror that is nearest to you (in terms of
network connectivity), and set the variable DEBIAN_MIR to contain
the URL of this mirror (including the trailing /debian). Browse the
sub-URL pool/main/d/debootstrap under $DEBIAN_MIR to locate a
recent version of debootstrap, and let DEBOOTSTRAP_URL contain the
full URL for this version."