1TB hard drive is only showing 500GB Capacity

Launch the Terminal app and run the following command so we can see the drive layout on your SSD, making sure to either post a screenshot of the output or pasting the results into the "Code Insertion" tool in the forum editing toolbar which has an icon that looks like "<>" as it helps to make it more readable:

diskutil  list  internal

Edit: Also, what is the make & model of the SSD? Where was it purchased? Keep in mind a lot of online vendors do sell fake & knock off SSDs. Run DriveDx and post the complete text report here using the "Additional Text" icon which looks like a piece of paper. This may help to determine a fake SSD and should provide the exact make & model, but it is still a good idea to tell us yourself so we can see if what you think you bought matches what you actually purchased.

The SSD looks Ok and looks legitimate, but the SSD shows having been powered on for 3,600 hours (or about 5 months 24/7).

Currently the SSD has a single partition of 500GB. Did you ever add a second partition to it possibly using BootCamp Assistant for installing Windows and later tried to remove it?

Have you actually had the SSD for at least five months? If not, then this might have been a used SSD (or one which had been returned). It is possible the SSD had existing partitions on it when you received it and only erased the one visible partition which is only half the size of the SSD itself. Or the SSD may have been provisioned to only show half its size.

You can try starting over again with this SSD by trying to erase the whole physical SSD. You will need to boot from a macOS installer (Internet Recovery Mode or bootable USB stick). Within Disk Utility you may need to click "View" and select "Show All Devices" so that the physical SSD appears on the left pane of Disk Utility. If you are using macOS 10.10 or earlier, then the physical SSD must be first partitioned and formatted instead. If the volume created is still just 500GB, then it may mean the SSD has a hardware size restriction on it, or the SSD is bad, or the SSD is fake (a good fake). Unfortunately I believe the only way to remove a hardware restriction of a real SSD is to reset the SSD to factory defaults (either using Windows or Linux boot disk...not entirely sure if that would work, or if it would require to manually readjust the size restriction...assuming a real SSD).

All of my observations support HWTech's suspicions. The information you provided clearly shows a 1 TB SSD but for whatever reason the APFS container is only 500 GB, leaving 500 GB unaccounted for.

If you were to boot Recovery and launch Disk Utility you should be able to resize that container to the SSD's 1 TB capacity. That is nondestructive and would be ideal. Failing that, erase the physical device. If that is not successful then I can only imagine it's simply not compatible with your model Mini. That is possible, though I doubt it. The only reason for mentioning it is that a 1 TB SSD was never one of that model's configurable options.

Try erasing the entire device and see what happens.

shadowsphere21 wrote:

Did you ever add a second partition to it possibly using BootCamp Assistant for installing Windows and later tried to remove it? — No. the SSD was directly installed into my Mac by an IT professional

Ask your IT professional about it.

Have you actually had the SSD for at least five months? Yes, I mentioned above I bought it 2019, and I had it installed as soon as it arrived from Crucial.

Sorry, while I have been sick recently and still tired & recovering, I don't see that information anywhere until you just mentioned it in the last post.

I can assure you it’s a real SSD and not a fake as you are assuming — it was bought directly from the Crucial website (I don’t know where this assumption is coming from)

This is the first time you confirmed where the SSD was purchased. I had to assume people are buying from anywhere...even Amazon where fake SSDs are known to exist. I did say I thought the SSD is legit, but without this information I couldn't be 100% sure.

It still doesn't completely eliminate the other possibilities I mentioned previously. Unfortunately the easiest way to attempt to fix a possible partitioning issue is by completely erasing the whole physical SSD as I mentioned previously, reinstalling macOS & restoring from a backup. Erasing the SSD will destroy all data on the SSD which has not been backed up. Unfortunately macOS and Disk Utility don't make it easy for the other options and sometimes the erase is the only option which will work.

One other possibility is a bad internal hard drive SATA Cable, but I've never seen one cause a problem with properly partitioning a drive....usually a bad cable results in performance issues or outright not working at all. A tech can test the SSD by removing the SSD and connecting it externally using a USB to SATA Adapter, drive dock, or enclosure to eliminate the internal cable being a source of the problem when attempting to erase the whole physical SSD.

You may need to contact your IT professional for assistance.

Other than the fact it's been somehow limited to half its capacity, before you condemn what appears to be a perfectly fine SSD please determine if the same lagging problems occur "Safe Mode": How to use safe mode on your Mac - Apple Support.

Capacity concerns notwithstanding, perhaps something else is going on with that Mini. Sometimes merely restarting a Mac in "Safe Mode" followed by restarting it normally fixes problems otherwise too tedious to diagnose.

Back up your Mac as a matter of course. To learn how to use Time Machine please read Back up your files with Time Machine on Mac - Apple Support. You will obviously need that backup should you decide to erase and reformat the SSD.

And I replied to the wrong person 😂

Please let us know how you eventually make out with this situation.

Good luck.