Multi-booting W7, XP and Ubuntu

Finally got Windows 7 Beta installed on the Dell E520. This certainly took a lot of time.
Initially I had thought the install happened because I had completely deleted Grub from the MBR. But, due to a very enlightening comment from somebody to my previous post here, I now believe it is much more likely (if not totally certain) that unplugging the external USB HDD was the main reason why I stopped getting that infernal error message and the install went completely smoothly.
I posted to the W7 forum on this both before and after the discovery of the influence of the ext HDD.
Just to strengthen this belief, while I was setting up a multiboot system on the Dell E520, I had to re-install W7. This time, Grub was in the MBR but the ext HDD was unplugged. The install went absolutely smoothly.
So, it seems the plugged in usb drive is what's causing the problem. However, why should this be? And how does this give rise to this error (Setup was unable to create a new system partition or locate an existing system partition)?
This is extremely baffling.
So, now I have Grub booting W&, XP and Ubuntu on the Dell E520. I'll just list here some other surprises I came across in doing this today:
1. Installing W7 give rise also to a 200 MB partition that seems to contain recovery and boot files.
2. After installing W7, booting just went straight to W7.
3. After this I installed XP. Now, only XP would boot.
4. When I installed Ubuntu with Grub going to the MBR, the menu.lst stanzas I included for W7 and XP were:

root (hd0,x)
chainloader +1

However, only (hd0,0) booted to anything, this being XP. What was surprising was that XP was actually on /dev/sda3 -- so how come this became (hd0,0)?
When I tried (hd0,1), I got a message about there being no bootmgr or "ntldr missing"
Don't know how this happened.
5. So, this is where I went ahead and re-installed W7. Now, both W7 and XP could be booted from the Grub menu although as before, W7 which is on /dev/sda2 booted when root=(hd0,1) while XP, which is on /dev/sda3 booted from root=(hda0,0)
6. As a comment, W7 was much easier to install that XP. With the latter, there was an awful amount of messing around looking for driver CDs, most of which have never been used since they were recieved. Indeed, with XP, I still haven't got sound working.