If you are running CD boot combined with DHCP, immediately after binding protocol to network card, the requester service is starting and contacting DHCP server. After contacting DHCP, it receives an IP address and tries to write it on the CD... of course it can't.
So, instead using DHCP in protocol.ini, try to use fixed ip address, in this case no information will be written on CDROM.
Replace the section in red from protocol.ini file:
[NTS$NTST2]
DefaultDomain=dag
BOOTPFLAG=DHCP
DriverName=ntsts$
VCs=16
VCReceiveLarge=6
VCSends=6
RcvWindow=2920
UseMemory=UMB
BINDINGS=NIC
LANABASE=1
with the following commands:
DNSAddress=[192.168.1.2] where 192.168.1.1 is your DNS server
DefaultGateway=[192.168.1.253] where 192.168.1.3 is the ip address for your gateway
SubNetMask=[255.255.255.0]
IPaddress=[192.168.1.3] where 192.168.1.3 is the fixed addres that you want to asign to your PC at the boot moment
I solved in this way a boot problem for a "SIMATIC IL77" computer which did not have floppy, but CDROM.