Whether it is 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 is 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 is a ZIP drive or a floppy, you have already got a DOS filesystem on it, you can use a command like this:
# mount -t msdos /dev/fd0c /floppy
if it is a floppy, or this:
# mount -t msdos /dev/da2s4 /zip
for a ZIP disk with the factory configuration.
For other disks, see how they are laid out using fdisk(8) or sysinstall(8).
The rest of the examples will be for a ZIP drive on da2, the third SCSI disk.
Unless it is a floppy, or a removable you plan on sharing with other people, it is probably a better idea to stick a BSD filesystem on it. You will 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(8) or /stand/sysinstall, or for a small drive that you do not 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/rda2 count=2 # disklabel -Brw da2 auto
You can use disklabel or /stand/sysinstall to create multiple BSD partitions. You will certainly want to do this if you are adding swap space on a fixed disk, but it is probably irrelevant on a removable drive like a ZIP.
Finally, create a new filesystem, this one is on our ZIP drive using the whole disk:
# newfs /dev/rda2c
and mount it:
# mount /dev/da2c /zip
and it is probably a good idea to add a line like this to /etc/fstab (see fstab(5)) so you can just type mount /zip in the future:
/dev/da2c /zip ffs rw,noauto 0 0