The Leopard installer requires a portion of free space following a system volume on your disk. When your disk was encrypted, that free space was used by a boot partition for the PGP BootGuard that allows you to login to your encrypted disk.
To upgrade from Tiger to Leopard or to re-install Leopard, your must first decrypt your disk. Your disk cannot be used by the Leopard installer if it is encrypted.
Use the follow steps to prepare the disk:
- Start the Terminal from the Leopard Installer (Utilities->Terminal).
- Type diskutil list.
- Identify your system disk. In most cases it is disk0s2.
There should be a volume called "Boot OSX" on the next partition, probably disk0s3. If your system is different than disk0, then use that identifier in place of disk0 for the rest of these instructions.
- Type diskutil unmountdisk disk0.
- Type gpt remove -i n disk0 where the n is the last number of the boot partition; for example, disk0s3 would be 3; you would type gpt remove -i 3 disk0.
- Exit the terminal.
The Leopard installer should now proceed without prompting you to erase your boot disk.
Later, when you encrypt your disk using PGP Whole Disk Encryption, the needed boot partition will be restored.