1. ls /Volumes reveals that it's "mjg", not "MJG"
2. Here's my testing procedure:
a. mount mjg using the Finder in my non-administrator account
b. login via Terminal to an administrator account.
c. sudo diskutil unmount /Volumes/mjg
d. I get a mini-manual for diskutil. (This happens if I use diskutil in the terminal in one of six ways -- (i) from my account, from an administrator account, (ii) using sudo first or (iii) not; and the same, only calling the volume "MJG".)
Disk Utility Tool
Usage: diskutil [mount(Disk)|unmount(Disk)|eject] <force>
[Mount Point|Disk Identifier|Device Node]
Mount, unmount or eject local disks or volumes.
force is only valid on unmount or unmountDisk.
Example: diskutil unmount /Volumes/SomeDisk
3. diskutil list doesn't produce anything that looks like the volume I'm trying to unmount. It's 135 GB in size.
/dev/disk0
#: type name size identifier
0: Apple_partition_scheme *55.9 GB disk0
1: Apple_partition_map 31.5 KB disk0s1
2: Apple_Driver43 28.0 KB disk0s2
3: Apple_Driver43 28.0 KB disk0s3
4: Apple_Driver_ATA 28.0 KB disk0s4
5: Apple_Driver_ATA 28.0 KB disk0s5
6: Apple_FWDriver 256.0 KB disk0s6
7: Apple_Driver_IOKit 256.0 KB disk0s7
8: Apple_Patches 256.0 KB disk0s8
9: Apple_HFS Themistocles 55.9 GB disk0s9
4. The graphical application Disk Utility, doesn't show mjg, either in my account or the administrator account. In other words, I couldn't unmount the volume using the graphical application if I tried.
5. Do points 2-4 add up to the following: diskutil doesn't see the volume I'm trying to unmount using diskutil? If so, I can see why there would be a problem.
Thanks. I hope I'm not being too dense. But I am learning a few things along the way ... I think.