With an operating system copied from a partition on sda (hd0 in grub) to a partition on sdb (external usb disk), can that system be made bootable?

My grub on sda will boot a system installed on an sdb partition, but it is not finding a system copied to another sdb partition by dd.

A rather indecisive googled comment suggests that it can't be done. There is lots of info on disk to disk coping but not on partition to bootable partition.

You'll apparently have to modify fstab and bootloader configuration. Other than that I don't see why it won't work.

In case this is MBR boot copying only the partition with dd will not boot - because the MBR is outside of partitions.

Exactly what command did you use to make this copy?

Grub may have a command "find" that may be used in some cases to find new installs.

A pretty easy way to correct it may be to install grub to this usb but syslinux is also useful but we still may need to know this command you used.

If this is MBR, copying a system from a partition on sda to a partition on sdb will require that you modify the boot file, grub.cfg if you are using Grub2. That is usually done by running grub-mkconfig or if you are using one of the Ubuntu's, sudo update-grub. There is no possible way the bootloader for either drive will be aware of the newly copied partition if you don't update grub. Not knowing which drive you are booting from or want to boot it from or whether it is UEFI or MBR means we can't be more specific.

Gentlemen, thanks for your help. To answer:
yancek: this is MBR, Grub2 on sda8 (debian jessie), modified by update-grub. When the grub screen appears, it shows the partitions on sdb (usb disk) as well as those on sda, but it does not boot the one copied by dd.

jefro: I didn't make a note of the dd command used. It was just a simple if to of with 4096 blocks.
My grub has no "find" command, but the grub "ls" command shows
It does not show (hd1,6) which is the OS partition copied by dd.
I do not intend to boot from the dd partition, but to have my normal grub boot it.

Emerson: the MBR on sda boots (thru grub on sda8) all OS partitions on sda as well as the one installed (not copied) on sdb1.

Matt Green: I modified the fstab on the dd partition, replacing the UUID that dd copied from sda1 with the UUID assigned to the dd partition.

The dd command would of overwritten the existing UUID with sda1's. Look at the output of the command lsblk. Are the UUIDs the same? To change sdb6 you need to use the tune2fs command which should match the one in /etc/fstab.

Does the new partition show up with all the other tools i.e. parted, fdisk etc...

michaelk: lsblk, blkid, fdisk, parted, as well as tune2fs -l UUID=<uuid of sdb6 partition> all show up sdb6. But Grub2 "ls" command does not find it. The attempt to load it via Grub2 says that it is not found and that the kernel has to be loaded first.

Curiously grub finds sdb2 and sdb4, neither of which are loadable systems. It will load the system on sdb1.