I have a HP DL380 G5 with 8x146GB single port SAS SSF disks (DG146ABAB4).
A couple of them have failed and I can't find any new single port disks anymore. They all seem to be dual port. I'm wondering if I can mix the two disk types? Are dual port backwards compatible?
Yes, the dual-port drives are fine for internal chassis use. In fact, that's pretty much all you'll find available now. HP also sends the dual-ported 6G drives as warranty replacements. They are entirely backwards-compatible.
Like ewwhite has said in the Accepted answer, it is possible to mix dual and single Port SAS disks in HP ProLiant Servers. Here's the official information from the HPE (Hewlett Packard Enterprise) web site:
http://h20564.www2.hpe.com/hpsc/doc/public/display?docId=emr_na-c01766210-1&lang=en-us&cc=us [ https://support.hpe.com/hpesc/public/docDisplay?docId=emr_na-c01766210-1 ]