Hi: Considering an iMac purchase if new offerings later this year appeal. Would be a 27", hopefully. Not 'handy' and would NOT try to open one & replace an internal HDD. If I bought one with a 512 gig or 1 tb internal SSD, and hooked up a thunderbolt external 4 tb HD, given that there are methods to combine 2 drives into a fusion drive, would that work? Would there be any disadvantage to this? Thanks. Richard.