you need a controller that supports partitioned disks. I believe Intel's controller does, but I'm not certain. you partition the disks such that you have 250GB partition and 1,750GB partition. with each disk partitioned thusly, you then create two raid0 arrays, one with the 250GB partitions such that they total 1TB, and one with the 1,750 GB partitions such that they total 7TB. if one disk dies, you lose the total sum of both arrays, and therefore, unless you have a solid backup solution for the 7TB partition and some great need to keep it separate from the 1TB partition, you're better off with just one large array. you could also create a raid10 or raid5 across the 1,750 for reduced capacity but increased fault tolerance, which would be my advice if you're insistent upon this approach.
Do you understand how to create raids... nothing of what you said will work... your talking about a form of software raid that has nothing to do with raid0, raid 5, or raid 10.... Its strickly just partitioning and extending drives... no raid involved... Which will do nothing but cause problems because there will be no form of backup or fault tolerance... Also... Partition tables are wiped when a raid is built unless done through software within windows(which can be risky/destructive) where as doing it through bios IS destructive(meaning full rebuild required, all partitions wiped) In other words... you would have to make the raid array, Then partition the drives within the raid
Please explain this, if im wrong, id like to understand how you do it... because every raid controller i have ever touched(which is a few thousand) has not had that kind of option or i have overlooked it