Those water cooled G5 machines were fun to work on (ie repair/refurbish/mod). I hung on to mine (and some Protools HD accl cards) a little too long and you can't give the stuff away now.
Anyhoo, if you're on Mac you can use Carbon Copy Cloner (still Mac only) to clone. It has safety net and scheduling features. CCC genuinely clones the whole drive including everything with the OS install. The clone copy will boot your machine just like the primary. A Time Machine volume will save your data and some app installs but will not boot your machine. You'd need to boot from a clone or other system drive, clean install OS to a new drive, then migrate your data from the Time Machine volume. Time Machine is a fine choice if this is starting to read like Klingon! It's simple and will save your stuff.
The power supply needing 3.5" external drives are more robust. The bus powered portable drives are more disposable. Or so it might appear.
Anyway, you're doing this right! You make backups. You replace broken drives and get backups current again. You avoid any scenario that might lead to needing to do data recovery.
My SOP here is to risk annoying anyone who might be listening rather than have to console someone who lost something.