If partitioning a drive with Disk Utility gives you unmount or resource busy errors, there may be workarounds you can use.
When trying to partition and format various USB hard drives, a number of users are getting errors in Disk Utility that states "Disk Erase failed with the error: POSIX reports: The operation couldn't be completed. Cannot allocate memory".
Tom and Rafe take a trip down memory lane, describing older, defunct software they used to love... and hate.
Troubleshooting the Apple Hard Drive updater (#4): Recovering your disk; Terminal can "see" damaged drive; more