Briefcase problem.

Hello, I'm posting this in this forum cause maybe it is related to user
accounts/rights.
I use briefcases to keep my files updated among my home and work PCs.
At home I use Vista but at work I use XP.
I created my briefcase in XP and put it in a USB HDD.
When I open my briefcase to work in any PC with XP it works perfect.
But if I want to work on my briefcase in a PC running vista, it takes a long
time to open.... I tried to copy a simple 90MB file from my briefcase to my
home PC (vista) and it told me that id would take an estimate 4hours
36minutes... and it took a long time ( I canceled after 1 hour).
I tried other USB HDDs and same problem.
I tried other PCs running vista ans same problem.
Any idea why this happen and how to solve it?

thanks
e.