you can also set grub partition with gdisk(8)
Command (? for help): t Partition number (1-4): 1 Current type is 'BIOS boot partition' Hex code or GUID (L to show codes, Enter = 8300): EF02 Changed type of partition to 'BIOS boot partition'
Important thing to note is that you CANNOT reuse your existing /boot or any other partition for this - 64KB might be enough (it should be just big enough to fit your /boot/grub/i386-pc/*.img files), but there is usually about 1MB free which is way more than enough.
If the partition was missing instead, you could have create New one, there is even usually enough space free before your first partition. For example, then it would be like:
# gdisk /dev/sdb Command (? for help): p Disk /dev/sdb: 7814037168 sectors, 3.6 TiB Logical sector size: 512 bytes Disk identifier (GUID): CEFE1861-C13E-4E30-AEC4-0037CA6CBE8D Partition table holds up to 128 entries First usable sector is 34, last usable sector is 7814037134 Partitions will be aligned on 2048-sector boundaries Total free space is 2014 sectors (1007.0 KiB) Number Start (sector) End (sector) Size Code Name 1 2048 3893247 1.9 GiB 0700 Microsoft basic data 2 3893248 12281855 4.0 GiB 8200 Linux swap 3 12281856 12806143 256.0 MiB FD00 Linux RAID 4 12806144 7814037134 3.6 TiB FD00 Linux RAID Command (? for help): n Partition number (5-128, default 5): First sector (34-2047, default = 34) or {+-}size{KMGTP}: 34 Last sector (40-2047, default = 2047) or {+-}size{KMGTP}: Current type is 'Linux filesystem' Hex code or GUID (L to show codes, Enter = 8300): EF02 Changed type of partition to 'BIOS boot partition' Command (? for help): p Disk /dev/sdb: 7814037168 sectors, 3.6 TiB Logical sector size: 512 bytes Disk identifier (GUID): CEFE1861-C13E-4E30-AEC4-0037CA6CBE8D Partition table holds up to 128 entries First usable sector is 34, last usable sector is 7814037134 Partitions will be aligned on 2048-sector boundaries Total free space is 6 sectors (3.0 KiB) Number Start (sector) End (sector) Size Code Name 1 2048 3893247 1.9 GiB 0700 Microsoft basic data 2 3893248 12281855 4.0 GiB 8200 Linux swap 3 12281856 12806143 256.0 MiB FD00 Linux RAID 4 12806144 7814037134 3.6 TiB FD00 Linux RAID 5 40 2047 1004.0 KiB EF02 BIOS boot partition Command (? for help): w Final checks complete. About to write GPT data. THIS WILL OVERWRITE EXISTING PARTITIONS!! Do you want to proceed? (Y/N): y OK; writing new GUID partition table (GPT) to /dev/sdb. Warning: The kernel is still using the old partition table. The new table will be used at the next reboot. The operation has completed successfully. 
