Is it simple to find the cause of parity check sync errors? All my drives are being reported as healthy and there's no I/O errors. My uptime is less than a day because I installed a disk to Preclear, which was running when the Parity Check started, The shutdown was clean.

zeus-diagnostics-20250701-1045.zip

Sync errors should never happen without a reason, run a correcting check, then run another one without rebooting, if new errors are found, there's likely an underlying hardware issue, I would recommend starting by running memtest.

Is it safe to run a correcting check while Preclear is running on a new disk?

Yes.

What does it mean if there's no errors the 2nd time without rebooting?

That the issue should be resolved, just keep monitoring future checks for more errors.