Just shut down AOLserver and PG first, then copy the relevant directories over to the new hard drive.
As to whether or not you can then unplug the drive and have it work on another system, well ... are you installing Linux on it? Or will it be a second drive in the new system with Linux already running on it?
And if it is a second drive, will it be mounted with the same path etc?
If you really replicate things precisely moving it should be a snap but you have to replicate a lot more than your OpenACS installation...
