New build - currently running Parity-Sync

My new server has two 8TB HDDs - one for parity and one for data.  I just got it setup and am running a parity sync right now, as that's what the Getting Started video told me to do.  Do I have to wait for it to finish before I can start transferring all my data over (using Teracopy) from my Windows machine?  Or can I just get started moving files over?

Also, I've read some things about "pre-clearing" a drive?  Do I have to do that?  If so, can I do it while this parity sync is running?

You can move data over now if you want.

Pre-clearing a drive just reduces the downtime for your server as it cannot add the drive to the array until it is precleared. Depending on the size, that can take hours, even days.  Your best bet is to use the Screen app, so you can preclear it while the server is up.  Once it's done, you shut the array down, add the drive, and it takes only minutes instead of hours/days.

You can't preclear a drive that is in the array, so it sounds as if you have already missed that boat. And a drive doesn't need to be clear unless you are adding it to an array that already has valid parity, so technically your data drives don't need to be clear.

Many people use preclear to test a drive anyway. If you want to do that you will have to stop the parity sync and unassign the drives. Of course preclear will wipe the disks so you will lose any data on them. If you decide to not preclear then after the parity sync you should at least look at the SMART for each drive to make sure they aren't having any problems.

It is possible to transfer files while doing the parity sync, but doing so will slow down the parity sync and slow down the data transfer. Best to start learning patience if you are going to be working with 8TB drives.

Very important! Set up Notifications so unRAID can notify you when there are any problems.

If you are only going by the Getting Started video I suggest you also begin to read some while you are waiting on things. The stickies in the forum are a good place to start.

And be sure to ask on the forum if you are ever unsure how to proceed. Seen plenty of cases where someone tried to fix something without really understanding what to do and only made them worse.