umounts and sync

[Samuel Maftoul]

(re-)hello everyone,
I'm currently writing a script for hotplug ieee1394 hard drives.
I'm encountering troubles with something:
my hotplug agent launch a script when you unplug the disk.
This script calls umount.
Am I going to loose datas (as the umount and stuff which are written on
the HD by umount won't be written because the disk isn't here anymore)?
Do you have a solution to avoid this ? 
mounting with sync option slows the disk.
I can also do a script: if no activity during 5 sec on the disk, do
sync. (Is it the "sync" command I need to do this ?)
As usually: Thanks for any help.
        Sam