I have one 2TB failed, have precleared the 4TB drive. At the same time I want to remove another 2TB drive which has pending sectors. What is the best way to accomplish that? Thanks!

Replace the failed 2TB drive with the 4TB and restore your data. When that has completed move the data from the other 2TB drive to your 4TB drive. When that drive is empty remove it from your array and then under Tools select the New Config option. This will rebuild parity based on your new disk configuration.

• • unRAID does not currently provide any supported way of removing a drive.  The only way you can remove a drive is to do a 'new config' and then redefine the array omitting the drive.
     
    The steps as I see it are:
    • follow the normal process for recovering a failed drive using the 4TB as the replacement.
      
    • Copy all the files off the drive you want to remove by copying between disk level shares (do not use a User share at this stage as it can lead to data loss)
      
    • Do a parity check to make sure that all your drives are OK (the next few steps are going to mean you run in unprotected mode)
      
    • Make sure you know what your current drive assignments (a screen shot is a good idea)
      
    • Stop the array and do a 'new config' from the Tools tab.  This will remove all current disk assignements
       
      
    • Reassign all the drives you want to keep in the array.  At this point it is a good idea to omit the parity disk.
      
    • Start the array and check that all the data disks seem to have come up fine.  If any come up as 'unformatted[' then you probably tried to assign the parity disk as a data disk.
      
    • If you are happy, you can now stop the array and assign the parity disk.  Starting the array after doing this will cause parity to be recalculated.  Until that completes you are running 'at risk' if another disk fails
      
    • When parity build has completed you should run a non-correcting parity check to ensure that the parity disk was written correctly.  This should show no errors
      

You may decide it is easier to simply leave the 2TB drive in the array and simply set up your shares to avoid using it for the time being.