Whether it's a removable drive like a ZIP or an EZ drive (or even a floppy, if you want to use it that way), or a new hard disk, once it's installed and recognized by the system, and you have your cartridge/floppy/whatever slotted in, things are pretty much the same for all devices.
(this section is based on Mark Mayo's ZIP FAQ)
If it's a ZIP drive or a floppy , you've already got a DOS filesystem on it, you can use a command like this:
mount -t msdos /dev/fd0c /floppy
if it's a floppy, or this:
mount -t msdos /dev/sd2s4 /zip
for a ZIP disk with the factory configuration.
For other disks, see how they're laid out using fdisk or
/stand/sysinstall.
The rest of the examples will be for a ZIP drive on sd2, the third SCSI disk.
Unless it's a floppy, or a removable you plan on sharing with
other people, it's probably a better idea to stick a BSD file
system on it. You'll get long filename support, at least a 2X
improvement in performance, and a lot more stability. First, you
need to redo the DOS-level partitions/filesystems. You can either
use fdisk or /stand/sysinstall, or for a small
drive that you don't want to bother with multiple operating system
support on, just blow away the whole FAT partition table (slices)
and just use the BSD partitioning:
dd if=/dev/zero of=/dev/rsd2 count=2
disklabel -Brw sd2 auto
You can use disklabel or /stand/sysinstall to create multiple
BSD partitions. You'll certainly want to do this if you're adding
swap space on a fixed disk, but it's probably irrelevant on a
removable drive like a ZIP.
Finally, create a new file system, this one's on our ZIP drive using the whole disk:
newfs /dev/rsd2c
and mount it:
mount /dev/sd2c /zip
and it's probably a good idea to add a line like this to /etc/fstab so you can just type "mount /zip" in the future:
/dev/sd2c /zip ffs rw,noauto 0 0