Can you mix drive sizes?
I have a 4-bay RAID with 4 x 3TB drives striped as RAID 0 using Apple RAID. I have a disk failing, and replaced it with a spare 6TB drive . But I still end up with a 12TB RAID, not 15TB. Can you combine different drive sizes in a SoftRAID and get that extra capacity?
No. In a formal RAID volume, all partitions are the same size. You could use the extra space as a non RAID volume, however.
Right, SoftRAID cannot handle this, and no stripped RAID can possibly handle this as every (sufficiently large) write must be striped to different partitions.
There are enterprise-level RAID systems that permit you to create volumes by logically concatenating "plexes", each of which provides a section of storage that can be RAID in different ways, so such systems could allow you to create a volume by creating (12 TB volume striped across 4 drives) concatenated with a (3 TB volume on one drive.) However, neither SoftRaid nor Apple RAID permit this kind of configuration.