For several years, the Mac tower CPU's have had PCI expansion slots, but due to the proprietary nature of the hardware, PC expansion cards don't just plug in and work. A few developers who understand the needs of Mac users have seized these opportunities to make sizable profits. One such development is a PCI expansion card capable of reading and writing to ATA-type hard disks. I've been using a PCI-ATA hard disk expansion card on my PowerMac 9500 file server for about nine months. In my setup, I do not use any slave drives.
RPM drives each of which is configured as a master.
The external drive tray can be populated with either of my two backup hard disks. It is important to note that the computer must be shut down before installing or removing a removable drive tray - failure to do so will result in serious hardware damage and loss of data. You need to turn the computer off before inserting or removing a hard disk drive cartridge. I can't confirm the damage or data loss since I always shut down before swapping out the removable drives. For me, backing up has become simple, fast, and requires no complicated software.