There was a company that offered a service where they upgraded your 4000/90 / 90a / 96 to support 64 meg SIMMs, larger cache and faster SCSI, so although they were likely rare, they did exist.Not up to 1/2TiB I'm afraid though, no matter how much one would love to. ;)
Oh! I missed those letters that make quite one heck of a difference :D John