MacBook Pro won't recognize external HDDs

• If a simple reboot of your Mac doesn’t fix things, see if the problem still happens in safe mode.

How to use safe mode on your Mac - Apple Support 

Safe mode can often correct weird software behavior by forcing the OS to do disk repair, clear caches and other housekeeping and temporarily disables third-party mods. When you're in safe boot, the machine won’t be at its best performance, especially with graphics, but that's expected.

Does the same thing happen while in Safe mode?

You can exit safe mode by restarting your Mac normally.

• Create a new user on the affected Mac and see if the problem persists. You can delete that user when you're done.

• Reinstall macOS from Recovery mode. Doing this doesn't have to affect any of your user accounts or data. Just don't erase, partition, format or otherwise alter your startup drive. Just launch the Reinstall macOS process after booting in Recovery. Always a good idea to have a recent backup of your computer before doing this.

How to reinstall macOS - Apple Support

Disk Utility is the Grand Arbiter between bare drives and Mounted Volumes. That is what you should be using to check for drives presence and condition.

My suggestion is to boot into Safe Mode, which generally does NOT load most third-party add-ons, and check while in Safe Mode. this strongly reduced added software as the culprit.

have you tried to recognize the drives while still in Safe Mode?

This is a puzzler to me. The fact that another Mac has no problem with these external drives suggests an issue with the port(s) on the older laptop or more likely with the USB-C dongle that you use as you suggested.

Have you tried using that dongle on on the newer MacBook?

And I'm sure you've tried using all of the different USB-C ports on the laptop, and none work with the drives any more?

Any chance you have another unrelated external drive that you can connect to the '19 Mac? (Grasping for straws here.)

I'm at a loss and recommend that you take the laptop to Apple for evaluation and repair.

You can make a Genius Bar appointment for hardware/software service and support using this link:

Contact Apple for support and service - Apple Support

Get your Mac repaired and see how much will it cost:

Mac Repair & Service - Apple Support

Call Customer Support  (800) MY–APPLE (800–692–7753)

or on line  https://getsupport.apple.com/

or call  AppleCare Support at 1-800-APLCARE (800-275-2273)

A drive that will not tell disk Utility its make&model and a reasonable non-zero size/capacity is the definition of DEAD. it can not be Mounted, accessed, repaired, or initialized in its current condition.

I used to say the things to check were:

• cabling and

• Power. but D.I. Johnson has just reminded me there may be software issues that could preclude this as well, so:

• Software issues

.

You're welcome so far.

It is a weird problem.

Please update us when/if you discover anything else that is helpful.