Buffers and underrun proofing can save the day, but the basic problem of having a writer on the same IDE cable that's the source of the write, is that they get in each others way... the old argument, that the speed drops to that of the lowest, is largely resolved by modern chipsets that support split timings for master and slave, but I think it did reappear for a while in early Ultra-ATA setups that had limitations on mixing Ultra and non-ultra speeds.
The safest way to configure the drives, and in part, my own bias in setting up, is:
HD as Primary Master
DVD Writer as Secondary Master ... that's the two most important drives.
CD-RW as Secondary Slave (don't try to use it and the DVD writer together)
DVD-ROM as Primary Slave (since using it with the HD, the HD can take the data as fast as it can give it).
That lineup should be OK for writing to either drive (but not BOTH) from the HD, and ok for an attemped "on the fly" from the DVD-ROM, so long as the HD is not working too hard - personally, I'd avoid on the fly copying, though at least with underrun proofing, it can survive a severe choke on the read speed... the first media I lost to that on an old drive, was the last time I tried it!