Hi.
It seems that SW RAID install on large drive cannot be managed by GRUB 'as is'
I'm also trying to setup a SW RAID fresh install with 2x 3Tb HDD in RAID 1 and i faced this (first) same problem, resolved with the following - on each drive :
a 100Mb partition dedicated to Biosgrub
a 200Mb partition in EXT4, not used, just with boot option
a partition for swap space, used as a RAID partition
a last partition for system (containing approx 3Tb) used as a RAID partition also.
Then, configure both RAID partitions for swap and / partitions, respectively.
Found on internet, this has allowed to pass this GRUB issue, but install is not working yet - i have other issues still open.
Hope this helps.