How many hard drives are in this Mac mini? I just purchased a Mac mini and the seller assures me that there are two 1 TB hard drives inside, with a total storage space of 2 TB (picture from the listing).
At first, I was under the impression that there was a second drive without an OS and that the partitioning was done on that (Windows-format?) drive. However, after erasing the accessible drive following Apple's instructions, and restoring my data with Target mode, I do not see a second drive listed anywhere:

If I start the computer pressing ALT I see
this.
In the Storage tab I see
this.
In Recovery mode the Disk
Utility lists this.
The diskutil list command returns this in
Recovery mode (more images).

I really, really don't want to have to go through this process to figure out if the seller is saying the truth about there being two drives inside, since it might prevent me from returning it, but I'd like to know if what I bought matches the item description.
Possible hypotheses:

there is no second drive
there might be a second drive, but badly connected/wired in so undetectable
there is a second drive that is blank / oddly formatted / undetectable

How can I know if there is a second drive in the Mac mini?
You have one drive.
Your diskutil list command tells you everything you need to know:  there's only one entry that has a &quot;physical&quot; 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 &quot;disk image&quot; 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 &quot;internal, physical&quot; because if the seller assured you &quot;there are two hard drives,&quot; you would have (in a Mac mini) two identifier with &quot;internal, physical&quot; 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.