BlogsDNA has recently posted a guide on how to solve the issue of hard drive partitions not showing up in the latest Windows 7 beta. For those with dual-boot or multiple OS configurations, this will restore access to XP, Vista, or other partitions for file transferring and other useful purposes.
Basically, all that needs to be done is the assignment of a drive letter for the XP/Vista/other OS partition. To do this, open the Start menu in Windows 7 and type in "diskmgmt.msc" which will open the Disk Management Interface, and assign a drive letter to the unrecognized partition.
More information, including step-by-step diagrams, can be found here