Due to the way IDE developed it is not very good at doing two things at once on the same channel.
Hence it is considered better when writing for the data to come from a device on a different channel.
Not really an issue backing up CD's anymore but with the larger data-rate required for DVD-Writing it still can be an issue.
As to cable not being long enough
i) Buy a longer cable
ii) Move DVD-ROM
iii) Move HD (buy a 3.5 to 5.25 inch bay adapter quite cheap these days.