There are four data storage modes on the CloudShell-2: RAID 0, RAID 1, SPAN and PM. This means that the hard disks will be detected separately in PM mode.
To select the RAID mode, set the RAID Mode DIP Switch to the setting that you want, then apply power while pressing the RAID Setup Key button until booting is complete.

RAID with different sized drives?

It can be done, however the raid size is restricted to the smallest drive in the array. It's generally frowned upon to use different drive types, let alone sizes for RAID, but it can be done if you don't mind sacrificing the extra space on your larger drive to do it. Your 3+4TB RAID would only be 6TB, and might suffer performance issues. If you have 2 3TB, and 2 4TB just combine the like sizes into two different RAID1 (or 0) arrays.

Is it possible to convert a raid 0 to raid 1 without losing any data? I am using 2 x 2TB HDDs and about 300 gig is currently stored on the array.

Unfortunately the answer is no. RAID 0 (striping) is currently giving you 2 x 2TB, whereas RAID 1 (mirroring) would only give you 2TB. Since you only have 300GB worth of data, you need to copy the data onto another disk (e.g. a USB drive). Once you change the RAID mode, you have to format the storages again.