[EDIT: see the next post, come back here only if that's not the issue]
OK but what did System Information show for that SATA drive? I expect nothing is there for that disk, but want you to check.
The test mount did not work because the /dev device for that disk is not there and so the device name mapping is different and when you rebooted the /dev/disk4 device is now the removable FLASH drive, which is already mounted so you get a resource busy error.
On the positive side potentially really messed up things like /dev not existing are not the issue, and it's not going to be an issue with diskarbitrationd.
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Just to confirm when you say "all internal" are you meaning this disk is attached to the motherboard SATA II controller or are you using a third party PCIe SATA III card?
And this is very important: Make sure you try shutting dow everything, physically powering off the computer and then powering it back on again, not just doing a shutdown or reboot from software.
In all these cases does the drive spin up even briefly when the system boots, try putting your fingers on it lightly to try to feel that happening given all the other noise during power up.
To be paranoid I would check the EFI us up to date
https://www.imore.com/how-upgrade-yo...c-pro-firmware and do a NVRAM and SMC reset.
https://iosafe.com/helpdesk/kb/faq.php?id=14
Try the same disk on a different SATA port, especially getting it on a motherboard SATA II port if you are using a SATA III PCIe card. Also simplify everything by unplugging (power and SATA cable) all other drives except the boot drive you are testing at that time and this "POOL 2" drive.
There can be issues around SATA versions in the cheesegraters (which could only apply if you are attaching to the motherboard SATA II), but I'm lost as to how OS versions interact with that. But that will be the next step to look at more. And if you are attaching to a third party SATA III controller there could be separate issues with the OS recognizing that... (which is why I want to see the System Info data for the SATA drive/controller).