ERR: No disks detected in Pioneer Pro

This may be a failing power supply.  The 6-drive legacy units' power supply has two 12V rails.  One provides drive power, the other powers the motherboard.  If the 12V rail that powers the drives was unable to handle the drive spin-up load, that would be the likely result.  But you might occasionally get through spin-up, and things could work (for a time).

Fortunately, there is an easy fix because it's a standard (though not particulaly common) power supply form factor.  You will find several threads in the forum about replacement options.  You can also use a standard ATX supply externally to verify that a power supply replacement will solve your problem or to power the unit while you backup files if you don't plan to repair..

Due to age, many units of the same generaton as yours are needing power supply replacement.  All things considered, they've lasted a good long while.  I've had to replace the supply in two of mine and have replaced one for a friend as well.