Make sure the external drive is formatted with a GUID partition map for your Intel-based Macintosh. Disk Utility will show you the type of partition map. Many external drives are formatted for PCs with a Master Boot Record partition map.
Assuming you have a Mac OS Extended (journaled) volume on a device with the correct partition map, check the disk directory of the volume to be split to create the eDrive, using either Disk Utility or the Volume Structures test in TechTool Pro. Also, run the Surface Scan test to check the drive for unremapped bad blocks.
A few people have found that they can create the eDrive after reinstalling TechTool Pro and repairing permissions. Neither step by itself solved the problem.
If you have files on the external drive already, make sure they are backed up before you attempt to create the eDrive.
If the volume on the external drive will not unmount, please try these instructions for using the lsof command:
If you boot your Macintosh from one volume and cannot unmount another volume, it is because the other volume has open files on it. You can use the "lsof" command to find the open files on a volume that does not unmount or eject. To do this, open Terminal (typically located in /Applications/Utilities) and type:
sudo lsof | grep
Leave a space after the "p" in "grep". Then drag the icon of the volume on which you want to locate the open files to the Terminal window. The pathname for the volume is pasted into the window for you. Press the Return key. Then, at the prompt, enter your administrator password and press Return.
Finding unexpected open files is frequently a problem with cloned volumes.
Each line of the output of the lsof command refers to one open file. The name of the process that has the file open is at the start of the line, and the complete pathname of the file is at the end.
Here is part of an earlier thread on this topic. The output of the lsof command in the following example consists of these three lines:
coreservi 68 root 10u VREG 14,32 5632 54 /Volumes/Maxtor_BU09/Desktop DB
coreservi 68 root 11u VREG 14,32 38082 55 /Volumes/Maxtor_BU09/Desktop DF
RetroRun 26448 root 25u VREG 14,32 214780244 11347 /Volumes/Maxtor_BU09/Main Backup/Maxtor_BU09.cat
You can ignore any references to DesktopDB and DesktopDF; the operating system can close them.
In the example above, the process named RetroRun shown on the last line (a process used by the Retrospect backup program) has a file named Maxtor_BU09.cat open on the volume Maxtor_BU09. The file is in a folder named Main Backup. The user quit Retrospect (either in the Finder or by using Activity Monitor, located in /Applications/Utilities) and the volume was then able to be unmounted.
Spotlight indexing can cause files to be open:
Spotlight consists of two processes, according to Bombich Forums moderator Andreas:
mdimport user process
mds root process
Makers of TechTool