vfs

The parameter you have to pass to boot from UUID is PARTUUID. So it should be root=PARTUUID=666c2eee-193d-42db-a490-4c444342bd4e.

The documentation explains why it‘s coming back with unknown-block(0,0):

kernel-parameters.txt:

    root=       [KNL] Root filesystem
            See name_to_dev_t comment in init/do_mounts.c.

init/do_mounts.c:

/*
 *  Convert a name into device number.  We accept the following variants:
 *
 *  1) device number in hexadecimal represents itself
 *  2) /dev/nfs represents Root_NFS (0xff)
 *  3) /dev/<disk_name> represents the device number of disk
 *  4) /dev/<disk_name><decimal> represents the device number
 *         of partition - device number of disk plus the partition number
 *  5) /dev/<disk_name>p<decimal> - same as the above, that form is
 *     used when disk name of partitioned disk ends on a digit.
 *  6) PARTUUID=00112233-4455-6677-8899-AABBCCDDEEFF representing the
 *     unique id of a partition if the partition table provides it.
 *     The UUID may be either an EFI/GPT UUID, or refer to an MSDOS
 *     partition using the format SSSSSSSS-PP, where SSSSSSSS is a zero-
 *     filled hex representation of the 32-bit "NT disk signature", and PP
 *     is a zero-filled hex representation of the 1-based partition number.
 *  7) PARTUUID=<UUID>/PARTNROFF=<int> to select a partition in relation to
 *     a partition with a known unique id.
 *
 *  If name doesn‘t have fall into the categories above, we return (0,0).
 *  block_class is used to check if something is a disk name. If the disk
 *  name contains slashes, the device name has them replaced with
 *  bangs.
 */

The last bit at the end says that if it cant understand the value, it returns (0,0), hence your error.

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Just to clarify UUIDs are the only reliable way for the
kernel to identify hard drives. There are two types: UUID, which is
stored in the filesystem and is not available to the kernel at
boot-time, and PARTUUID, which is stored in the partition table and IS
available at boot time. So you have to use

root=PARTUUID=SSSSSSSS-PP

as /dev/sd?? can change with devices plugged/unplugged.

Don‘t forget to capitalize the hexadecimal number SSSSSSSS-PP you get from blkid!

The more easy to use

root=LABEL=
root=UUID=

only work with an initramfs that fetches these identifiers.

So, if you use a non-empty initramfs, you can have all three! With an empty initramfs, you only have PARTUUID.

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