I don't know about the 4400, but I know that the 6320 has to be backward-compatible with the EIDE spec. Most drives are backward compatible. I have a Maxtor 20 ATA/66 running in my 5400, and it had a Western Digital 30G ATA/100 running in it before that. Check the manufacturers' web sites to see if they're backward compatible with the earlier ATA specs. There's some information over at XLR8yourmac.com, and I've actually seen an explanation of the IDE specs, with reference to performas, someplace on the Web. Use Sherlock and it might pop out.