boot - Grub no longer recognizes Windows 7 after installing Windows 8

Yesterday I installed Windows 7 (on /dev/sdb1) and Windows 8.1 (on /dev/sdb2).

"Of course", my existing Ubuntu 14.04 (on /dev/sdb3) was therefore rendered unbootable. I fixed this with the Boot Repair utility via Ubuntu 14.04 Live USB.

However, something odd is going on:

1. I ran Boot Repair after I installed Windows 7, which nicely made Ubuntu and Windows 7 show up in the grub menu.


2. I ran Boot Repair again after I installed Windows 8.1, which nicely made Ubuntu and Windows 8 appear in the grub menu...But...where has Windows 7 gone?

So I ran Boot Repair again, to no avail. I manually ran

$ sudo update-grub

which resulted in

Found linux image: /boot/vmlinuz-3.13.0-35-generic 
Found initrd image: /boot/initrd.img-3.13.0-35-generic 
Found linux image: /boot/vmlinuz-3.13.0-34-generic 
Found initrd image: /boot/initrd.img-3.13.0-34-generic 
Found linux image: /boot/vmlinuz-3.13.0-24-generic 
Found initrd image: /boot/initrd.img-3.13.0-24-generic 
Found memtest86+ image: /boot/memtest86+.elf 

Found memtest86+ image: /boot/memtest86+.bin
Found Windows 8 (loader) on /dev/sdb1
done.

weird - It shows Windows 8 on /dev/sdb1, where Windows 7 is installed.

Also:

$ sudo os-prober 

/dev/sdb1:Windows 8 (loader):Windows:chain
$

...no Ubuntu? No Windows 7?

I also ran Boot Info Script:

               Boot Info Script 0.61      [1 April 2012]

============================= Boot Info Summary: ===============================

=> Grub2 (v1.99) is installed in the MBR of /dev/sda and looks at
sector 1 of
the same hard drive for core.img. core.img is at this location and looks
in partition 112 for .
=> Grub2 (v1.99) is installed in the MBR of /dev/sdb and looks at sector 1 of
the same hard drive for core.img. core.img is at this location and looks
in partition 112 for .
=> No boot loader is installed in the MBR of /dev/sdc.

sda1:

---

File system:       swap
Boot sector type:  -
Boot sector info:

sda2:

---

File system:       Extended Partition
Boot sector type:  -
Boot sector info:

sda5:

---

File system:
Boot sector type:  -
Boot sector info:
Mounting failed:   mount: unknown filesystem type ''

sdb1:

---

File system:       ntfs
Boot sector type:  Windows Vista/7: NTFS
Boot sector info:  No errors found in the Boot Parameter Block.
Operating System:  Windows 7
Boot files:        /bootmgr /Boot/BCD /Windows/System32/winload.exe

sdb2:

---

File system:       ntfs
Boot sector type:  Windows Vista/7: NTFS
Boot sector info:  No errors found in the Boot Parameter Block.
Operating System:
Boot files:        /bootmgr /Windows/System32/winload.exe

sdb3:

---

File system:       ext4
Boot sector type:  -
Boot sector info:
Operating System:  Ubuntu 14.04.1 LTS
Boot files:        /boot/grub/grub.cfg /etc/fstab

sdb4:

---

File system:       ext2
Boot sector type:  -
Boot sector info:
Operating System:

Boot files:

sdc1:

---

File system:       ext4
Boot sector type:  -
Boot sector info:

Operating System:
Boot files:

sdc2:

---

File system:       ntfs
Boot sector type:  Windows Vista/7: NTFS

Boot sector info:  No errors found in the Boot Parameter Block.
Operating System:
Boot files:

-- snip because the rest is personal :) --

Boot flags are on /dev/sda1 and /dev/sdb1.

So everything seems in order, but Grub simply doesn't recognize the two Windows correctly...What's going on?

(For completeness, /dev/sda is a broken and unused SDD which I'm experimenting with.)