Identify the MacOS X version on your Recovery HD partition
There is an easy way to check which version of Mac OS X is on the Recovery HD partitions.
Since you can have such a partition on each attached Mac-formatted disk, it's interesting to know if they are all up-to-date.
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To check for any Recovery HD, open Terminal (enter 'Terminal' in a Spotlight-search and you'll see the application listed) and enter the following command:
$ diskutil list
This will produce output which can look like this:
/dev/disk0 (internal, physical):
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *500.1 GB disk0
1: EFI EFI 209.7 MB disk0s1
2: Apple_HFS Macintosh HD 499.2 GB disk0s2
3: Apple_Boot Recovery HD 650.0 MB disk0s3
/dev/disk1 (internal, physical):
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *1.0 TB disk1
1: EFI EFI 209.7 MB disk1s1
2: Apple_HFS Data HD 999.9 GB disk1s2
/dev/disk2 (external, physical):
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *8.8 TB disk2
1: EFI EFI 209.7 MB disk2s1
2: Apple_HFS DROBO 8.8 TB disk2s2
The important one here is the third partition of the first disk. We can now simply mount that partition and find out which OS X version is installed there.
Enter the following commands:
$ diskutil mount /dev/disk0s3
Volume Recovery HD on /dev/disk0s3 mounted
$ grep -A 1 -i productversion /Volumes/Recovery\ HD/com.apple.recovery.boot/SystemVersion.plist
<key>ProductVersion</key>
<string>10.11.6</string>
$ diskutil unmount /dev/disk0s3
Volume Recovery HD on /dev/disk0s3 unmounted
There you have it!
And since a screenshot speaks a thousand words:
Some interesting information on installing / updating a Recovery HD partition when you find out yours has an older version of Mac OS X on it, or no Recovery HD partition at all: