Boot from the tiny internal hard drive with the SCSI CDROM connected.
If you can't get this to work, even though you could boot from the hard drive by itself, earlier, then there is a problem wtih the CDROM or the cableing or how it is configured.
If you can boot from the hard drive with the CDROM attached, load the Mac OS CDROM while booted from the hard drive. You may need a CDROM driver, although if your MacOS install disk is bootable, it should have one loaded on it. The disk may need to be inserted at boot time for it to load, though. My memory is hazy.
Anyway, if you need a driver/extension, the one that shipped in Apple System 7.6 had generic support for all drives (removed in 7.6.1) or use something like CDROM Toolkit from FWB or some such, if you have it available. Oh, I think I have Speedy CD here: https://www.prismnet.com/~trag/SPEEDYCD.SEA.hqx .
Okay, now, if you can boot from the hard drive, and get the CDROM to mount, then go to the Start-up Disk control panel, choose the 7.5.3 CDROM as the startup disk (if it's not available, but it is mounted, it might not be a bootable disk) and restart the computer, leaving everything turned on. The machine should now boot from the CDROM without any keyboard commands necessary.
If you can't get this to work, even though you could boot from the hard drive by itself, earlier, then there is a problem wtih the CDROM or the cableing or how it is configured.
If you can boot from the hard drive with the CDROM attached, load the Mac OS CDROM while booted from the hard drive. You may need a CDROM driver, although if your MacOS install disk is bootable, it should have one loaded on it. The disk may need to be inserted at boot time for it to load, though. My memory is hazy.
Anyway, if you need a driver/extension, the one that shipped in Apple System 7.6 had generic support for all drives (removed in 7.6.1) or use something like CDROM Toolkit from FWB or some such, if you have it available. Oh, I think I have Speedy CD here: https://www.prismnet.com/~trag/SPEEDYCD.SEA.hqx .
Okay, now, if you can boot from the hard drive, and get the CDROM to mount, then go to the Start-up Disk control panel, choose the 7.5.3 CDROM as the startup disk (if it's not available, but it is mounted, it might not be a bootable disk) and restart the computer, leaving everything turned on. The machine should now boot from the CDROM without any keyboard commands necessary.