Problem Installing Ultrix on DECstation 2100

(apologies to those on the DecTec list, looking to widen the audience) I am struggling to surmount a problem installing Ultrix 4.5 from a CD-ROM onto a DECstation 2100 (MIPS) After choosing the type of installation (basic or advanced) it tries to offer the system disk selection. However, it keeps failing because it is detecting the CD-ROM drive and saying it has an invalid block size. I can hear it spin up the hard disk that I want to install on, so it should be finding a suitable hard disk, but it looks like it keeps choking on the CD-ROM drive. I can't tell if it is failing to see the hard disk properly, but when I tried NetBSD on the same disk, and in the same machine, it is fine, so the hard disk shouldn't be the problem. Any ideas on how to resolve this? Thanks Rob