The server has 2 partitions as in the following example of
plesk sbin sysinfo df-full command output:
# plesk sbin sysinfo df-full
534461288448 8114397184 526346891264 /dev/sda2 /
534461288448 8114397184 526346891264 /dev/sda2 /var/lib/docker/devicemapper
Plesk detects only one disk at Tools & Settings > Server Information.
This is Plesk bug with ID #PPPM-7483 that is planned to be fixed in future Plesk updates.
Currently, there is no workaround for this issue.