leave it on the default mode for an array disk, so 'Read/Write' it's only UD drives that you have to mess with the mode.
simple but lengthy of course, expect a long wait, just ensure you obey the rule (see last paragraph).
you can do a parity check, but it shouldn't be necessary unless you suspect parity is not in sync with what is written to the array, the choice is yours. If you want to move data off another array drive (i assume the 6tb is part of the array right?), then it should be as simple as selecting everything on /mnt/diskX and copying to /mnt/diskY, where diskX = 6TB and diskY=18TB.
One VERY important thing to mention here, NEVER copy data from a disk share to a user share, or vice-versa, you must always copy from a user share to a user share, or from a disk share to a disk share, if you do not obey this rule then you may experience data loss.