Simple backup questions

[unixjingleman]

Hi There
To backup a FreeBSD file system to an external usb hard-drive the command is:

#dump -OLuf /path/to/external/drive /dev/ad0sla

right?
The only thing is that i backed up my MacBook first, so i think it is formatted for HFS+. So do i need to re-format the drive in order to back up FreeBSD filesystems?. I also want to back-up my OpenBSD filesystems on the same drive. However the only system that i have with a windowing system, and GUI utilities is the MacBook, at the moment. So do i need to format this external drive or should i leave it un-formatted. if i need to format it what should i format it as?.
If I'm just using it for storage do i need to format it?. Can i just enter the above dump command to put it on the HFS+ formatted drive?.
I'm sorry to ask so many questions. I've just never backed up whole systems before.
Thank you for your time, and any replies you might send
regards unixjingleman