I have a 500GB MyPassport drive I use to clone my Mac’s internal drive, it’s plugged into one of the USB ports behind my Apple Cinema Display. After I eject/un-mount the drive I don’t want to unplug and re-plug the USB cable to re-mount the drive, I can re-mount it in Disk Utility, but that’s still kindda annoying, so I looked and here’s how you do it in the command line.

When you have the drive mounted, do a df to see what the device is:

[ProBert:~] ayn% df
Filesystem    512-blocks      Used Available Capacity  Mounted on
/dev/disk0s2   976101344 427449800 548139544    44%    /
devfs                220       220         0   100%    /dev
map -hosts             0         0         0   100%    /net
map auto_home          0         0         0   100%    /home
afp_0TT7Xo1noyS900m5Am0Pyetl-1.2d000017 1949330784 1365867776 583463008    71%    /Volumes/Time Capsule
/dev/disk2s2                            1949330720 1365933248 583397472    71%    /Volumes/Backup of ProBert
/dev/disk1s3   976510944 427839352 548671592    44%    /Volumes/AYN MBP Clone

The device name is /dev/disk1s3 in my case. After that, you can mount and un-mount in Terminal easily by doing this:

[ProBert:~] ayn% diskutil unmount /dev/disk1s3
Volume AYN MBP Clone on disk1s3 unmounted
[ProBert:~] ayn% df 
Filesystem    512-blocks      Used Available Capacity  Mounted on
/dev/disk0s2   976101344 427449672 548139672    44%    /
devfs                221       221         0   100%    /dev
map -hosts             0         0         0   100%    /net
map auto_home          0         0         0   100%    /home
afp_0TT7Xo1noyS900m5Am0Pyetl-1.2d000017 1949330784 1365867776 583463008    71%    /Volumes/Time Capsule
/dev/disk2s2                            1949330720 1365933248 583397472    71%    /Volumes/Backup of ProBert
[ProBert:~] ayn% diskutil mount /dev/disk1s3
Volume AYN MBP Clone on /dev/disk1s3 mounted
[ProBert:~] ayn% df -h
Filesystem      Size   Used  Avail Capacity  Mounted on
/dev/disk0s2   465Gi  204Gi  261Gi    44%    /
devfs          111Ki  111Ki    0Bi   100%    /dev
map -hosts       0Bi    0Bi    0Bi   100%    /net
map auto_home    0Bi    0Bi    0Bi   100%    /home
afp_0TT7Xo1noyS900m5Am0Pyetl-1.2d000017 1949330784 1365867776 583463008    71%    /Volumes/Time Capsule
/dev/disk2s2                            1949330720 1365933248 583397472    71%    /Volumes/Backup of ProBert
afp_0TT7Xo1noyS900m5Am0Pyetl-1.2d000017  930Gi  651Gi  278Gi    71%    /Volumes/Time Capsule
/dev/disk2s2                             930Gi  651Gi  278Gi    71%    /Volumes/Backup of ProBert
/dev/disk1s3   466Gi  204Gi  262Gi    44%    /Volumes/AYN MBP Clone