Target Mode via external HDD?

I assume you backup at your neighbor's because you want off-site backup (a very good idea) in case something happens to your home, or there is a theft at your home, etc...

How about each of your running CrashPlan and specifying each other as the backup location. This mode of CrashPlan is free. You backup your stuff to his Mac over the internet (securely) and he can backup his stuff to your Mac again over the internet. Of course you can do it one-way, if you neighbor doesn't want to backup to your Mac. The data is encrypted before leaving your Mac, so your neighbor cannot read it. Of course you have to make sure you do not forget the encryption password you use :-)

-OR-

You could use an external disk. Backup up your data to the external disk. Take the disk over to your neighbors. Get the disk back, backup again, take it back, repeat.

-OR-

Get 2 external disks. Backup. Take disk to neighbors. Return with the other drive for the next backup. Backup and exchange drives again. repeat.

-OR-

If you do not want to have off-site backups, then just backup to and external disk and keep it in your home.

Backup software options are many. Built-in Mac OS X Time Machine. SuperDuper (excellent). Carbon Copy Cloner (equally excellent). Others.

Create a bootable clone. SuperDuper or Carbon Copy Cloner very well recommended. Then if your boot drive refuses to boot, you can boot holding the "Option" key and select the bootable clone external drive.

At this point you can choose to use the clone to repair the boot drive, if that is possible. Use the clone to restore the boot drive from a Time Machine backup, or if the clone is sufficiently recent, restore the boot drive from the clone (use the same software that made the clone).

If the boot drive has failed, you of course would get it replaced, then restoring would occur as above.

If all you really wanted was the ability to get an OS running on your Macbook Pro, but had another source for restoring data, you could create a bootable SD Card (I'm assuming your Macbook Pro is sufficiently new that it has an SD Card slot on the left side next to the USB ports. You could use something like Carbon Copy Cloner to selectively clone just the OS and some key Applications and Applications -> Utilities files to a 16GB or 32GB SD Card.

While booting an SD Card will not be fast, it can be an emergency bootable device.

But no matter what, you should use the external to make regular backups.

You might want to even consider making a deal with John to backup critical personal files via CrashPlan, and allow him to backup critical files of his own to your Mac via CrashPlan. That way the most important stuff is at least safe from fire and theft. Again using CrashPlan this way is free.

No matter what you should backup regular and often.

For example. Every day when I go to work, I use Carbon Copy Cloner to make a backup over the network to my Mac at work (they are sitting next to each other). It is not bootable, but it is all my data. I also use CrashPlan at home to a Mac mini in the basement. My work iMac does SuperDuper backups to external disks every night (although most of the really critical stuff I use my iMac for are on company servers, or I have replicated to company systems because I use it across multiple systems.

The point is backup. My backups have saved me many times since I started in this business back when programming was done with 80 column punched cards, magtape, and disk drives that were the size of washing machines :-)