Hello everyone ..

I long wondered that if you only lose 20% instead of 33% due to parity info in RAID5-arrays when using like 6x1TB disks, can you just create the 3x1TB array first, transfer everything you want backuped from the 2 other disks to the newly created array and grow it to 6 and will it use 20% of parity if you grow it from 3 to 6 instead of creating a 6 raid-array right away?

I have 1TB disk entirely filled so I would like to create a "small" raid first of 3 and then expand to 6.

The purpose of the RAID-array is to store files intact so the checksum matches everytime, disk load will not be very high since it will only perform uploads with maximum 1MB/s, but I want to be able to read it meanwhile.

Actually, if you read here: http://en.wikipedia.org/wiki/RAID#Standard_levels you'll see that RAID 5 uses distributed parity across ALL disks present, but can survive 1 disk failure.
I wouldn't worry about percentages. Just build the array anyway you like, so long as you have min of 3 disks to start.

Personally I wouldn't trust the only copy of my data to be on the array I am growing..

If it were me I would probably build the new array using the 5 new 1TB disks, transfer the data to the new array, then assign the old 1TB disk as a hot spare...