Maybe you've recently got a red dot on one of your drives, and you want to take it out of the array. You've got enough extra space, and don't need to replace the drive. Here are the following steps to shrink the array without losing any data.
In this example Disk 2 is to be removed.
1. On web management page for configuring user shares, remove disk 2 from inclusion in any shares (or exclude it) (eg. disk1,disk2,disk4 -> disk1,disk4)
2. open up MC , and browse to the /mnt/disk2 folder on both windows. rename all the primary share folders (eg movies/series/music) to movies_ / series_ / Music_
3. once done, open the renamed folder on the left, and your destination disk / user share on the right, and move the files inside over using F6. Should not encounter any duplicates any more. Repeat as neccesary for all share folders
4. Once all files is moved (verified via main page that the drive is almost empty ( mine had max 30MB on it), you still need to reset the array configuration(utils -> new config), and rebuild it excluding the affected drive (keeping your parity drive exactly the same drive as before...)
Do keep in mind - your system is currently at risk, and will only be free from data loss once step 3 is completed
unRAID simulated the disabled disk using parity and ALL THE OTHER DATA DISKS. Even though the physical disk is disabled, you can still read and write to it.
/mnt/user/whatever is just another view of /mnt/disk*/whatever. The files you see are EXACTLY the same, as /mnt/user is just a view of the files (another way of listing them, somewhat similar to linked directories or shortcuts in windows) This view is still in place even when a single physical disk is disabled.
thanks to Neo_x, and Joe L. on this forum post