Expand RAID5 array or switch to RAID10

[George Duffield <forumscollective@gmail.com>]

Hi

I have a RAID5 array comprising four 3TB drives and I've basically got
100GB remaining free so it's time to expand the array.  With this in
mind, and considering my requirement is predominantly to increase
capacity, I'm wondering whether it'd be best to add another 3TB drive
and expand the array whilst retaining RAID5 or to switch out to a
RAID10 array.  My thinking is 1) adding a 5th 3TB drive to the
existing array may result in drive failure during the rebuild; and 2)
RAID10 is very expensive to implement.  Hence, I'm tempted to consider
other options (recognising that whilst it'd be convenient I don't
actually need everything stored on a single array).   One such option
would be running two raid5 arrays comprised of 3x 3TB each yielding
12TB of storage across the two arrays.


Questions:
+ Is my assumption re RAID5 drive failure correct/ likely?
+ Is there a non-destructive way to migrate from RAID5 to RAID10?
+ 2 x RAID5 arrays seems pretty appealing - from a reliability and
cost effectiveness standpoint - yes/no?