I always configure my disks with two root partitions for just this reason. The built kernels live in a /boot partition by themselves in the first few cylinders of the disk. /usr/src is also common.
On 12-Jul-2002 Keith Watson wrote:
When making significant changes to an existing system (e.g. kernel builds or major software changes), what backup* strategies do you adopt to protect yourself from knackering the whole thing? I know this will depend a lot on what hardware is available but is there a commonly accepted approach? [snip]