Lets say I have a RAID 5 setup with 3x 4TB drives, and 1x 3TB drive- if I eventually replace the 3TB with 4TB, could I increase the volume capacity?
You cannot increase capacity of a volume until all the disks are replaced.
All partitions in a RAID volume must be the same size. (Or it is not really RAID, but something different)
I have a 4 x 1TB Drive Thunderbay 4 and I replaced one of them to a 6 TB disk but you need to replace the other 3 drives to take advantage of the available space. question is how does it know that the disks I replaced are new ones?
Thanks
SoftRAID does not know (nor care) what disks you use, as long as they are reliable.
Once you replace all the disks you can "resize volume". Sometimes you need to perform that command twice to get the Finder to update.

