Well, all indicators for both disks are OK (no warning, failing or failed) according to DriveDX. A have also ran short self-test (do not have licence for full test) and it was also completed without error.
Using Disk Utility or Apple System Profiler, get the drive designator for the drive you want to test which will be in the form of "disk0". You will need this in a minute.
In the Finder, Right-click (or Control-click) on the DriveDX app and select "Show package contents". Then navigate to "Contents/Resources". Open the Terminal app and type "sudo " without the quotes, then drag & drop "smartctl" onto the Terminal to auto populate the path to the actual tool. Then type the following without the quotes " -t long /dev/diskN" where you replace "diskN" with the drive designator for the drive you want to test. Here is an example using "disk0" where "0" is a zero:
sudo <path-to-DriveDX.app>/Contents/Resources/smartctl -t long /dev/disk0
To view the results of the selftest, you would use replace the "-t long" with "-l selftest", where "l" is a lowercase "L". Example using "disk0":
sudo <path-to-DriveDX.app>/Contents/Resources/smartctl -l selftest /dev/disk0
It may be a good idea to check the selftest log first so you know what is already listed there since it may not show the current test until it is completed. It will likely take 4+ hours to test your 3TB drive, even longer if the system is actively using the drive.
If the SSD supports the selftest you can also use this method to run the extended or long test on the SSD.
Note: These commands will prompt you for your user password as the commands need administrator privileges to run.
Like I said the information provided by DriveDX does alert users to many types of drive failures, but not to all of them so one of the drives could still have an issue not easily detected even if the selftests pass which is mostly a surface level scan of the blocks/sectors.
Unfortunately APFS is still a very new filesystem so it really isn't surprising to see these types of issues. Plus the necessary documentation needed to create utilities to repair APFS has not been released by Apple yet so third parties are unable to update their tools to work with APFS.
