Installing Multiple OS on external hard drive

I don't see why you should disconnect the internal hard drives, probably just a precautionary measure. Recently I did a install of Win 7 on my laptop hard disk and Ubuntu + openSUSE on the External one, without disconnecting and what not. I chose to install Win 7 bootloader on the internal drive, and the Ubuntu one on the external drive.

Here's my suggestion:

1. Boot from Windows XP CD.
2. Point it to install onto the external drive. { you will have to goto the "advanced" mode during installation/partitioning }
3. Next boot from Windows 7 DVD.
4. Windows 7 will detect the Windows XP installation and will ask if you wish to upgrade, select no, goto clean install mode and let it install on the internal drive. Windows 7 bootloader will be written to the internal drive, and should give the option to boot Windows XP & Windows 7.
5. Boot from Linux Mint and install it on the external drive. Choose to write grub onto the external drive.
6. Finally boot from Ubuntu 10.04, and choose to install it to the external drive and the bootloader to the external drive ( here, again, I believe just before the install starts, you'll have to select advanced mode and select to install the bootloader to the external drive).
7. Finally, select the first boot device from the BIOS as the external hard disk ( USB, I guess). This is because,since Ubuntu was installed last and the bootloader was written to external drive, grub will detect all OS's - Mint, XP, Win 7 and Ubuntu and you can choose which to load.

Your partitions will be something like

• NTFS partition of reasonable size on the internal drive for Windows 7
• NTFS partition of reasonable size on the external drive for Windows XP
• A fairly large NTFS partition, where you can store all your documents, media, downloads et al - and this will be common across all OS
• An ext3/ext4 / partition for Linux Mint
• An ext3/ext4 / partition for Ubuntu
• A common /swap partition of type swap for both Ubuntu & Mint
• A common /home partition of type ext3/ext4 for both Ubuntu & Mint

You can select partition sizes as you deem fit, since you haven't mentioned the sizes I cannot say for sure how much you might require.