Q: internal hard drive no longer detected

Your profile says PowerMac. Power Macs use a Power PC processor and can only go as high as 10.5.8.

Mac Pros have Intel processors

OS X: About System Information and System Profiler - Apple Support

- Does it show up under SATA/SATA Express in Systrem Information/Profiler?

- Try moving the HD with sled to another drive bay that is known to work

If it is an External drive, the most frequent failures are of the Interface card in the enclosure. Try another enclosure.

The poster said "One of my internal hard drives seems to have vanished. "

Grant Bennet-Alder wrote:

 

If it is an External drive, the most frequent failures are of the Interface card in the enclosure. Try another enclosure.

Thanks lllaass,

I stand corrected.

Try another bay but I think the drive is failing/failed and needs to be replaced.

A disk recovery app can likely recovery at least some data