Microsoft recomends agains using online file stores for PST file storage.
http://blogs.technet.com/b/askperf/a...n-t-do-it.aspx

There is a long delay and data can get corrupt. Even in a corporate environment with lots of fast file shares we see issues when PST file get large and are are on the network. Keep all PST files locally. A thunb drive is a good option if you need mobility.