[ *SNIP* ]
I guess that gparted's confusion is because of the nested partitions.
Another cause could be the GPT and APM debris. (It is the main job of
make_isombr_part to remove this.)
In order to check the theory about nested partitions you could delete
partition 2 which you don't need for BIOS booting.
E.g. by this interactive run:
$ sudo fdisk /dev/sdb
... welcome messages , maybe use p to get the table shown ...
Command (m for help): d
Partition number (1,2, default 2): 2
Partition 2 has been deleted.
Command (m for help): w
The partition table has been altered.
Syncing disks.
Maybe gparted likes it better afterwards.