the main difference between hdd and ssd is acces time..
and the other factor is about transfer rate..
Since hdd is mechanical base, it takes time when try to find and accessing sector of files..
but ssd do it almost instantly..
that's why ssd makes your computer very responsive to when you executing programs..
when you raid hdd, okay, you'll double up the transfer rates...
it also make access time gettin worse, since there are 2 process that try to find and accessing sector of filesprocessing in system..and it also takes time to merging data from both hdd..
That's why storage raid, actually makes your system less responsive than single storage..
or just buy a single SSD and one HDD and set them up in a raid, money problems?