Is there a way to preclear a new disk on Windows (or any OS except for Unraid), such that I have a more or less seamless integration of the new disk when I add it to the array? My server is in a remote location and I will only have access to it for a few days.

Unfortunately, there is no Windows ( or any other OS) program that will do what the various versions (and incarnations) of preclear scripts do.  However, I do believe that there is a way to run a VM inside of Windows and you could run a instance of Unraid on that.  (You would probably want to use a Trial version license unless you wanted to purchase a license for it.)

 

If all you wanted to do is to thoroughly test a new disk, you could run the testing program from the HD vendor.  (I usually try to get 70-100 hours on a disk to weed out early failures before adding it to the array.)   Be sure to look at the SMART results before and after to determine if the disk's quality looks suspicious.  Most people look at SMART attributes  5, 187, 188, 197, and 198.  They should all be zero before and after the test. 

 

I recognize why you want to preclear the disk prior to your trip out to the the server's location.  Unraid will first clear the disk by writing all zero's to the entire disk.  With larger disks this process can take longer than 24 hours.  Once this is finished, you have manually start the format process to add it to the array. 

 

You could install the 'Unraid Connect' plugin which I understand has a remote access feature.  you can find more details here:

 

        https://docs.unraid.net/connect/help/

and

        https://docs.unraid.net/connect/remote-access/

 

Hope this helps...