Please note that this is for the “legacy” version of GRUB still widely in use, not the next-generation GRUB 2. If unsure, check out this guide on how to find out which version of GRUB you are using. For the GRUB 2 version of this guide, click here.
GRUB has a different system for naming drives than you’re used to in Linux, and you will need to understand this before proceeding with commands or customisisation that requires this specific information. Both the drive and the partition are numbers in GRUB, and starting from 0 (zero).
This is different from the Linux convention of naming a drive a letter, and its partitions as numbers, starting with a and 1 respectively (eg: “a” in sda1 meaning first drive, with the “1” meaning the first partition on that drive). Also note that the drive and partition are separated with a , (comma) in the designations GRUB uses.
So therefore the first partition on the first hard drive (sda1 or hda1 in Linux) is hd0,0 in GRUB. Similarly, your second hard drive (sdb or hdb) would actually be hd1, and if you were talking about the 7th partition on your 3rd drive (sdc7 or hdc7) it would be hd2,6.
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