‘Compaq diagnostic partition’ is how certain partitioning tools label an MBR partition with type ID 0x12, because a partition with this type ID has historically has been used by Compaq to host their diagnostic tools, so that they could be started without an extra floppy disk. The type ID was different from that used by normal (then FAT16) partitions so that the partition wouldn’t be visible when using the operating system normally.
Though the mentioning of Compaq is misleading, the way Lenovo laptops use this partition type is pretty similar. On my Lenovo G550, a partition of this type hosts a small NTFS file system with label LENOVO_PART
, containing a Windows Recovery Environment that I can boot into to run somewhat similar, if more relevant to the current age, diagnostic tools. This Windows RE environment uses these properties (partition type ID and volume label) to identify the diagnostic partition, so changing them may disrupt those diagnostics’ operation.