Debian Installation :: Dual Boot UEFI - Grub Not Recognizing Drive

I've set up a dual boot system with Debian and Windows 8, both installed on their own drive, with their own boot partition. I installed eveything in UEFI-Mode with fast- and secure boot turned off. Both installations are working, as I can access them by changing the boot priority in the Bios. What I cannot achieve is to let grub boot my windows installation.

This is the output of parted -l:

Code: Select allModel: ATA Samsung SSD 840 (scsi)
Disk /dev/sda: 128GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt

Number  Start   End    Size    File system     Name  Flags
 1      1049kB  512MB  511MB   fat32                 boot
 2      512MB   111GB  111GB   ext4
 3      111GB   128GB  17,0GB  linux-swap(v1)

[Code] .... 

As you can see, my linux install is on sda, my windows install on sdc (sdb beeing a data disk). This is the entry I made in the 40_custom file in etc/grub.d:

Code: Select allmenuentry "Windows 8.1" {
insmod part_gpt
insmod chain
set root='(hd2,gpt2)'
chainloader /EFI/Microsoft/Boot/bootmgfw.efi
boot
}

I think this should be fine, but if I choose the windows entry wehen grub is booting, it says: error: no such partition. It's my first debian installation, and I am stuck here. Not too much of Linux experience in general.