The finder listing for the contents of my HDD doesn't add up. Where is the extra data?

Thanks very much for the advice, M. Johnson. I felt that might be the problem — so I have taken it.

Before erasing the disk, I thought it was advisable to do a full back-up — so I cloned the smallish drive using SuperDuper.

This has led to further questions regarding drives and drive structures.

The (external - Seagate) partition I cloned to was formatted as Mac OS - Extended (journaled).

I now see that the original internal drive is APFS. (Presumably, the previous owner reformatted it as such before selling it on - since the machine is pre-2016.)

So:

1) Should I format the drive again as APFS? (ie., no major advantage to "case sensitive," etc.?)

— I note that, on "Erase Disk" there is, in fact, no HFS+ or OS Extended option even available

(slightly saddening for, at one point, I was thinking of reverting this tiny old museum piece to an earlier operating system, such as Snow Leopard: partly for sentimental reasons (I loved the old iTunes and might use it almost exclusively as a juke box cum video streamer); partly because my guess is that the older OSes hog considerably less drive space, so might run faster and more efficiently?

— do you know of any way I might be able to erase and reformat as HFS+ / OS extended should I wish to some day?)

2) When I repartitioned my Seagate to back up the HDD, should I also have formatted that as APFS?

— ie., is APFS better than Mac OS - extended (journaled) — or is it really more of an SSD thing?

3) Could that difference in file system explain why, now, I can't even boot from the external clone?

— I use System Preferences - Start Up Drive: it whirs on a black screen for an unbearable time; when I return, I see it has has eventually just booted from my internal HDD (the one I am about to erase).

Thanks again for your insights,

Eric

(Ha! I love it that Apple has interpreted my use of the Latin tag, meaning "with," as an unrepeatable expletive and replaced it with three asterisks! It won't even allow me to edit my question. Does Apple fear people may use this forum to hunt for inappropriate videos?)

(Also BTW, I hesitated to presume you were either Mr Johnson, Mrs Johnson, or Ms Johnson - which is why I used the single M. as a polite title. I regret that I can't, now change that to just D.I. Johnson.)

Ah! One brief answer to the second part of my question 1).

You can revert an APFS drive to HFS+ should you wish to, by jumping through a couple of hoops:

https://dreamlight.com/how-to-reformat-an-apfs-drive-as-hfs-with-disk-utility/

(It did allow me to edit my third comment above - but not to delete this now superfluous reply box)