How many hard drives are in this Mac mini?
Solution 1:
Possibilities are:
- There is no second drive
- There is a second drive that is undetectable because faulty or badly connected
You can find out which it is by simply opening the black plastic bottom and looking, no "process" required.
In the picture of what you will see when opening the back, I have indicated the two connectors for the two possible disks with red arrows. One will already be occupied by the disk we know is in the Mac mini. If the other one is also occupied, then it is the second alternative: there is a disk but it is faulty or the connection is faulty. If it is unoccupied there is no second disk inside.
Solution 2:
You have one drive.
Your diskutil list command tells you everything you need to know: there's only one entry that has a "physical" characteristic to it and it's disk0.
When you boot into Recovery, there are many RAM disks that get created and in your case they are disk1 through disk19. If you notice on each identifier, it describes it as a "disk image" as in the example below:
/dev/disk1 (disk image):
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme +2.1 GB disk1
1: Apple_HFS OS X Base System 2.0 GB disk1s1
What you want to look for is anything that says "internal, physical" because if the seller assured you "there are two hard drives," you would have (in a Mac mini) two identifier with "internal, physical" descriptors like that of disk0:
/dev/disk0 (internal, physical):
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *1.0 TB disk0
1: EFI EFI 209.7 MB disk0s1
2: Apple_HFS Macintosh HD 999.3 GB disk0s2
3: Apple_Boot Recovery HD 650.0 MB disk0s3
What the seller showed you was a (Bootcamp) partition. When booted, the partitions appear to the OS as their own separate drives.