How to replace a drive in FreeNAS 11.2 using the wizard.

First we need to remove the bad drive from our pool.
1. Login to the wizard UI.
2. Go to Storage->Pools->Select a Pool->Click the Gear Icon->Select Status
3. Pick the drive you wish to take offline, click the 3 dots and select Offline.
4. Turn off your system so you can replace the drive.

Remove the physical disk
1. Now physically remove the drive you took offline.
2. Then install the new drive.

Adding the New Drive to your Volume and Resilvering.
1. Power up your system.
2. Login to the wizard. From the Dashboard your pool should show one less disk than before.
3. Go to Storage->Disks
4. You should see the new disk and in the Pool column it should show: Unused. (It is not assigned to a pool). Note it’s name, ie ada3 (or whatever for yours).
5. Go back to Storage-Pool->Click the Gear Icon->Select Status
6. You should see your old drive as OFFLINE. From the 3 dots select Replace. From the popup asking you the Replacing disk just select the disk, you inserted, ie ada3 and click Replace Disk.
7. FreeNAS should then start resilvering which is rebuilding the RAID array. This can take a long time so be patient. (A 10 TB drive can take a day or more).
8. In the top right it will show resilvering with an icon with 2 arrows spinning in a circle.

Links:
https://www.youtube.com/watch?v=mb_1pKI398Y

Simple mirroring (RAID 1) is my RAID choice for home FreeNAS setups.  My choice is a 3 drive mirrored RAID.  Quick summary below.

Why Mirroring?

• Simple
• Capacity is the same as a single drive. 
• The more disks the more reliability.
• Read speed is faster due to round robin reading from the mirrored drives.
• A 3 way mirror gives you 3x read performance.
• Write speed is the same as a single drive.
• Fault Tolerance via mirroring.
• A 3rd drive in a mirror gives you a hot-spare that is already synced.
• More Flexibility — Upgrading the pool with mirroring is easy, just zpool add.

Why Not RAID-Z?

• Most home RAIDs won’t have 4, 5, …, 8 or more disks.
• Generally Slower Read Performance than mirrored
•  Must access all disks for every single block of data, no gain, speed limited to the speed of a single disk.
• Corner Case: Streaming large amounts of data in parallel can be faster than mirrored, but this is not normal for home users.
• Write speed is the same as a single drive.
• Fault Tolerance — a higher probability that we lose data
• More disks means higher probability for a failure.
• Disk Failures means you need to get a replacement quickly, more quickly than a mirror.
• Less Flexibility — generally once your set your RAID it’s set.
• It can be changed but it’s complicated.

Reference:

• resilvering – the act of recreating data on a drive added to the pool.
• RAID-0 – basic striping. All disks work as 1 giant disk, no fault tolerance.
• MIRRORING / RAID-1 – basic mirroring.  Every disk has the same data.  The more disks, the more reliability.  Minimum of 2 disks required.
• RAID-Z / RAID-5 – n-1 space, 1 parity disk for fault tolerance. 1 disk failure, before you lose data.  Reads and Writes about the same speed as a single disk.  Minimum of 3 disks but 4 recommended.
• RAID-Z2 and RAID-Z3 – same as RAID-Z but with 2 or 3 disks for pairty.  This equates to 2 or 3 disk failures before you lose data.  Minimum of 5 and 8 disks respectively.
• RAID-10 – RAID 1 + 0.  Mirrors then stripes data.  Minimum of 4 disks required.

Links:
https://constantin.glez.de/2010/01/23/home-server-raid-greed-and-why-mirroring-still-best/
https://jrs-s.net/2015/02/06/zfs-you-should-use-mirror-vdevs-not-raidz/
https://www.ixsystems.com/community/threads/mirroring-vs-raidz2-resilvering.43230/