Raid Array with only a Partition of some Disks

Solution 1:

Firstly don't use RAID 5 at all, it's very well documented how dangerous it is, especially with large slow, especially consumer, HDD's - it's been considered dangerous for over a decade now and it frankly upsets me seeing it still being an option on disk controllers and even in software.

Anyway what you want to achieve isn't usually possible with hardware disk controllers, you could probably make software RAID work like this but it wouldn't be advisable, I'm sure it'd be fragile and hard to rebuild when the inevitable disk failure happens.

Just buy the extra disks to run this as a R6 or R10, knowing you have a supportable and stable platform.