From reading the word 'slave drive', and 2TB as storage drive, yet you have both sda5, sdb4 and sdb1 as OS on the storage drive....hmm
that leads me to guess you have a ide ( PATA) drive for your 'sdb'. just guess
2TB should be a sata drive
and you might be seeing inconsistency in drive designation each time you boot one of the three OSes..
may be you can confirm
1. you have a mix of pata and sata drive
2. boot a few times over each of the three OSes, then each time do two commands on terminal, record the terminal output, then compare all three sets of them to see, the only thing you should see are these three sets are IDENTICAL.
if there are not, then you are in for very inconsistent issue for booting,,, then you works on grub become nothing but confusing..