> What is the different between /dev/cd0a and /dev/cd0d?

It's not a complete explanation, but the info you want is referred to in
the first paragraph of the man page on the cd driver (man 4 cd):

   The cd driver provides support for a SCSI CD-ROM (Compact Disc-Read Only
   Memory) drive.  In an attempt to look like a regular disk, the cd driver
   synthesizes a partition table, with one partition covering the entire CD-
   ROM. It is possible to modify this partition table using disklabel(8),
   but it will only last until the CD-ROM is unmounted.  In general the in-
   terfaces are similar to those described by wd(4) and sd(4).

In the case of the "regular disk" interfaces, the "d" partition is the whole
disk, and the "a" partition is often the root filesystem if it's the disk
containing the OS. Can't tell you what this difference would be for CDs,
though. Maybe you could do something interesting with multi-session CDs?

	- Joel