/dev/sda, /dev/sdb, /dev/sdc are the disk drives as the Linux file system sees them. /dev/sda1, /dev/sda2, /dev/sdb1 etc. Are the partitions on said drives, again as Linux file system sees them.
However, GRUB does not work with those. Rather, it calls the disks hd0, hd1, hd2 and so on, in the order that BIOS sets them up for boot. The digit after the comma specifies the n-th partition on that disk (so, for example, (hd1,0) would indicate the first partition of the second disk in boot order, as numbering starts with zero).
Have you checked the boot order in your BIOS and made sure that the disk you want is actually recognized, and the third one in boot order? You might want to try, for instance, making the Ubuntu disk first one in boot order and do
from the GRUB prompt.
Good luck
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