Add One or More New Data Drives
You already have an unRAID array up and running and you want to expand your storage by adding one or more new data drives.
- Stop the array
- Power down the server
- Install your new hard drive(s)
- Power up the server 
The new drive(s) will show up under "Unassigned devices."
Next you need to pre-clear your new drive. You don't have to pre-clear the new drive, but if you don't, unRaid will automatically "Clear" the drive, which takes the same amount of time (I think) and keeps your entire array offline until it completes = lame.
Get the preclear_disk.sh from here. Follow the instructions on that page.
JoeL doesn't mention another program called Screen, but I highly recommend running the preclear_disk.sh script from a Screen.
Once you get the "Nerd Pack" plugin, you need to choose which pieces of Nerd Pack are activated. Go to your main unRaid GUI > Settings > Nerd Pack. Scroll down and tick Screen, then Apply.
That should install the Screen, along with another dependency.
Using Screen is pretty straight forward. Here's a quick tutorial.
Once you're in a Screen, and run the preclear_disk.sh, I also recommend using the email (which needs some setup) me when it's done report. The command looks like this:
./preclear_disc.sh -m email@example.com -M 3
===============PLUGIN BELOW IS BROKEN AS OF 8/28/2016=================
As of unRaid 6, there's a terrific pre-clear plugin.
- Visit the official plug in page here
- copy the XXXXXX.plg link, and paste it into unRaid menu > Plugins > Install Plugin tab, hit Install
- The pre-clear plugin also says that it does not come with the actual pre-clear script. You'll need to follow those directions to download that script and get it copied to the correct folder.
- Stop the array.
- On the "Devices" assignment page, assign the new disks to available slots.
- Go back to the main unRaid management page, if the drives are new, or if they have any other file-system other than reiserfs, they will appear as "unformatted" You will need to check the checkbox to format them. this will erase any data that was previously on them by first clearing them and then partitioning and formatting them with a reiserfs file-system.
When this operation finishes, all the data disks, including the new one(s), will be exported (shared) on the lan and be available for use.
The format operation consists of two phases. First, the the entire contents of the new disk(s) is cleared (written with zeros), and then it’s marked active in the array. Next, a file system is created. unRAID Server uses the ReiserFS journalled file system.
The clearing phase is necessary to preserve the fault tolerance characteristic of the array. If at any time while the new disk(s) is being cleared, one of the other disks fails, you will still be able to recover the data of the failed disk. Unfortunately, the clearing phase can take several hours depending on the size of the new disks(s). Formatting, once the drive is cleared, usually only takes a few seconds.
The capacity of any new disk(s) added must be the same size or smaller than your parity disk. If you wish to add a new disk which is larger than your parity disk, then you must instead first replace your parity disk. (You could use your new disk to replace parity, and then use your old parity disk as a new data disk.) You can either swap their positions physically on the cables and leave the drive assignments the same on the devices page, or logically swap their assignments on the devices assignment page.
 Sometimes your computers' bios will try to be helpful and boot off the new disc. With unRaid being the primary operating system, and on a flash drive, this is not helpful! Re-do the boot order in the bios. boot order
 When you run preclear_disk.sh from a regular terminal window, in the case of a headless server, the process will cancel if the connection gets interrupted for any reason. If it cancels, you'll have to start over. As the process can take 24+ hours, it's better to run via Screen. This way, you can exit the terminal, and the process will continue to run in the background.