I'm using Digidesign ProTools Mix Plus3 system with 4 SampleCell cards.
Yeah...sounds like you're using some fairly uncommon hardware & software.
Since you have 4 SampleCell cards, you obviously need a computer with with 4 PCI expansion slots. Also, I know that your original question revolved around SCSI & ATA hard drive compatibility.
Here's 5 suggestions I can think of:
1. You could purchase another used 9650 or 9600 Macintosh computer (I bet you could find one on E-Bay cheap). I did an e-Bay search, and there are 2 Macintosh 9600's for sale currently.
2. Depending on what's wrong with your 9650, you could fix it. Usually it's something fairly easy to fix/replace like the power supply. Unless you spilled a cup of coffee on it, and shorted everything out! ;D
3. You could purchase any other older Macintosh computer that runs OS 9, has 4 PCI slots, and uses SCSI hard drives.
4. You could install your SCSI hard drive into an external hard drive housing...that converts from SCSI (internally) from SCSI to USB.
5. I did a Google search for "SCSI ATA conversion" and came up with the link below. You could check out these products to see if they will work with the G4 computer your considering. You could do further search's as well.
SCSI to IDE/SATA/USB/FW Adapters - PC PitStop
I hope that this helps,
- Nick
p.s. By the way I wanted to add...it sounds like your "main issue" is the inability to reinstall the software that's on your SCSI drive because you do not have the original disks. I hope you do understand that ALL hard drives eventually fail!!!
When your SCSI hard drive finally "craps-out"...you're really going to be in a boat-load of trouble!!! Because you will no longer be able use your software!
So it might be a good idea to be continuously searching for a set of original disks for the software.