I have a Win7 box with an Ubuntu 10.10 VM accessed in Virtualbox.
The Ubuntu install has the ex4 filesystem. Can I access the shared folders in both Windows and Ubuntu without corrupting the data?
I made this mistake when dual-booting my laptop and made changes to a folder in Ubuntu. When I tried accessing that data in Windows, the access time was very slow, basically about 10% of normal. The only way I could fix it was to restore the data from backups and not make changes in Ubuntu.
If I do this with the shared folders, will the same thing happen?
What you're reporting is very strange. I almost always use the same "downloads" and "documents" folders from both Windows and Ubuntu, and have never experienced slow downs from either OS. I have kept both of these directories on an external hard drive that is formatted as NTFS.
Additionally, I have used Ubuntu VMs from within Windows in the past, and still use a Windows VM from within Ubuntu, with shared folders on all of them, but never experienced any discrepancies in access times.
Why you ran into such a problem before is beyond me, but like "Centaurus A" suggested, try copying a few files over at first, and if it seems OK, then you know you're safe. If not, then you might want to ask around a bit more about why the slow down happens in the first place and how it can be prevented, as it isn't supposed to happen.
Don't see why there would be a problem; however, it would be easy enough to test by copying a few files to a shared folder. If the shared folder versions are problematic you would still have the originals.