Catalina killing my 500GB SD storage

Ok, there's the issue. Something went amiss with the installation. Your Mac should not have multiple "Macintosh HD - Data" volumes ... only one. We have seen this on a number of posts here.

At this point, you should make sure that you have a good backup. Then you have a few choices:

1. Use Recovery Mode to first completely erase the drive, and then, install a clean copy of macOS Catalina. (recommended)
2. Attempt to delete the extra Macintosh HD - Data volumes. This can be performed with the Disk Utility in Recovery Mode.

You're very welcome! I'm glad that worked out for you.

According to Apple macOS Catalina requires a minimum of 12.5 GB of storage space to run. Curious, did you upgrade to Catalina or performed a clean install? If the former, did you notice the 90% "loss" immediately after the upgrade? For the latter, was the loss after the clean install or after you restored your data?

APFS has an unusual way of handling storage. That a portion is greyed out does not mean that is used. Every write to SSD takes a hit on its usable life, so APFS minimises the writes, but confuses expert and novice alike in the way it is presented to us.

If you truly need to resolve the anomaly you can force Spotlight to reindex. I did and my grey space became "vacant" again

Ok, looks like Lurkums has addressed this.

If you were running macOS Mojave before, most likely, the OS reformatted your Mac's system drive for APFS. When you then performed a clean install of macOS Catalina, the OS further created two volumes that you can see in the image you provided. The first volume, Macintosh HD, is read-only and contains the basic macOS operating system files. The second volume, Macintosh HD - Data, is read/write and contains some common system files, but mostly your apps and data.

Both of these volumes reside in a single APFS container. Think of the container analogous to the older partition concept. The advantage of using containers is that it allows for multiple volumes to reside inside of it. Yet each volume can grow (or shrink) as required. This is not something volumes could have done with the older operating systems. The "hashed" areas represent that "growth" area.

Bottom line? Catalina is NOT "killing" your system drive, it is actually using it more efficiently.

The "Storage" panel can be a bit vague.

Open up Terminal and enter the following line followed by a return

and copy and paste the results:

diskutil list

It will give a fine breakdown on what is going on with the drive.

Also, you should not be seeing the two "drives" in that panel.

The command that woodmeister50 has provided you should result with something like:

/dev/disk0 (internal):

#: TYPE NAME SIZE IDENTIFIER

0: GUID_partition_scheme 500 GB disk0

1: EFI EFI NNN.N MB disk0s1

2: Apple_APFS Container disk1 500 GB disk0s2

/dev/disk1 (synthesized):

#: TYPE NAME SIZE IDENTIFIER

0: APFS Container Scheme - Physical Store disk0s2 500 GB disk1

1: APFS Volume Macintosh HD NNN.N GB disk1s1

2: APFS Volume Preboot NNN.N MB disk1s2

3: APFS Volume Recovery NNN.N MB disk1s3

4: APFS Volume VM N.N GB disk1s4

Does it?

Try this: Start in Safe Mode then restart normally.

Use safe mode to isolate issues with your Mac - Apple Support

Comparing yours with mine I see some startling differences:

To my untutored eye, your Ids 5 and 6 are odd, at least compared with my internal drive