Backup file got larger and larger!?

I bought an external 60 GB USB (2.0) Hard Drive
(partitioned into 2 partitions) to"back up" the 30 GB
HD in my Dell SX270. I used the OS' (Win2000 Professional)
"backup" command.
The 1st backup seemed normal, and I backed up
fairly frequently. Finally, the last backup failed
because of "insufficient hard drive space". The
primary (internal) hard drive has only about 10GB
used, so why has this "backup" policy consumed
the entire 30GB partition?
Since I assume the backup file is not correct,
I deleted it (it was a single filename). It also
seems that the SX270 treats the external HD
as a "tape drive" in some message I received.
I'd like the partition on the external HD to be
able to restore the system if the original HD
fails, and I put in a new, unformatted one.
TIA