2012 Mac Mini Fusion Drive Setup error 69888

[scoobyrox]

Ran into one problem when I used the initial coldStorage command in Terminal. I kept getting "error -69888: couldn't unmount disk". I saw someone else had this same problem. However, no one indicated the solution. After some trial and error, I did solve the problem. Easiest way is: 1) restart computer while holding Command and r to go into recovery mode. Go to Disk Utility. Format both drives that you want to fuse together. Then exit Disk Utility and go into Terminal in the top menu area. You can do the formatting in terminal but you need to know the syntax to do that. So I prefer using the GUI Disk Utility. Now if you do 'diskutil coreStorage create nameOfFusionDrive disk0 disk1', it will be smooth sailing for the rest of the steps. disk0 and disk1 may change. It depends on what you see when you do 'diskutil list'. I hope this info will help someone else that may run into the same problem. REMEMBER: you need to backup your original data since this process requires drive formatting.