Just installed two 1TB drives in the UnRAID box, and wanted to set them up in a RAID0 configuration.

First step is to move the data off the current cache drive (250GB SSD). This involves moving the data to the HD array by setting the Share settings for “Use Cache” to “YES” from “Only”. Then click the “Move Now” button on the Main page. I had to do this for both the “appdata” and “domain” shares.

All that’s required is to select the two devices as cache devices, start the array. If there are multiple cache drives selected the system will automatically set them up as RAID1. This can be confirmed by clicking on the cache drive and looking in the “Balance status” section. Data, System and Metadata will all show RAID1.

Because btrfs is very clever, the cache drive RAID array can now be converted to something else, which is a RAID0 array in my case.
In the “Balance” field enter “-dconvert=raid0 -mconvert=raid1” and click the “Balance” button.
This will convert the array.
The Balance Status will reflect this change, with Data now being RAID0 and the storage about is increased.

To move the data back to the SSD cache, set the Share settings for the user shares for “Use Cache” to “Prefer” and then click the “Move Now” button. This will move the data back to the SSD cache and off the HD array.

Reference 1, reference 2, reference 3