Desktop has died with a mobo fault
So remove HDD
Replacement machine is a compact HP built in such a way that a HDD swap is not feasible.
It has Ubuntu 11.4 basic distro installed.
The 'rescued' drive is fully uptodate
The idea is to attach the donor as an external and copy all over.

Is it doable?

What snags am likely to find?

you could use some form of DD to perform the replacement of the new internal drive on the HP, but i would suggest a fresh install of Ubuntu get it setup the way you wish, then just rsync the data you need back from the old, now external HDD, to the new internal drive in the HP.

This will be cleaner and provide a much more stable environment as the new install will be built 100% against your new hardware. this is not MS Windows land were it would crash, but you will have better optimized drivers and modules this way in your kernel.

i hope that helps.

Ubuntu 11.04 hasn't been supported for a year so you might be better off doing a backup and a new install. I did this with cp -ax a couple of years ago and everything seemed to work. I think there are better ways of doing it but don't have any suggestions as this is just a hobby for me and I lack the knowledge. Unless the old and new computers are close to identical in hardware, you will have some configuration to do even if the copying does work. Hopefully, someone will have a better option for you. dd or rsync?? You will probably need to configure your bootloader. You haven't indicate what if anything else is on the newer machine.

For Ubuntu, I would copy the /home from the external to a separate partition on the new system. Then do a full install of a current Ubuntu, taking care to specify the /home partition, and not to format it. Accept all the users it finds, and set the password(s) appropruiately.
When I used Ubuntu I did this on all upgrades - but generally they were within a couple of years. You may be ok too, but if not, nothing lost by trying.
You may need to re-install any packages you specifically installed - in need you can get a list of these by chrooting into the external.

Thank you all for your words of wisdom and support
Hep
I note the 'no longer support for 11 Ubunto- but on another much older machine withe same distro,after many warnings telling me 'no longer supported coming in from presumably Canonicle an invitation to up grade came in.
Naturally accepted with alacrity- it took about three hours to grind n on the very ancient Toshiba Satellite 100 ,but it gfot there.
The only hang up it seems to be router code does not want to 'stick' and has to be entered every boot up ,several times too be for connection is achievedd.
But the may be hardware-hardworn,because booting and getting the drive which is new-to kick in.
But its all fun.
For interest and have in my collection an even older Toshiba Portage running CrunchBang.
A/very compact and fast disro for older machines.

I also look after the machines of some older folk ( I'm 81) who are perplexed at what to do about the passing XP-whilst there is nothing/for them to do --it still works I saw it as an opportunity to introduce them to the world of Linux,
Here I installed 'Pinguy' which delighedcthem so far-as a dual booot install,they can look in at XP if they so desire but it seems this happens less frequently as they getvused to the new way.

J