Time Machine backup external disk not mounting anymore

I just installed Security Update 2015-002 (Version 1.0) on my MacBook. The latter has a 1.0TB internal hard drive, backed up onto a 2.0TB external USB2.0 3.5 inches hard drive. However, after mentioned update, when I start the computer, I have the following message:

The disk you inserted was not readable by this computer

And I have 3 options:

• Initialize...


• Ingore


• Eject

Clicking on Initialise... brings up Disk Utility, where the external Time Machine disk is now seen as "2 TB External RAID Media" unformatted...

I would like to avoid having to reinitialize it as I have many years of backup history on it. Strange what a "simple" security update can do. I tried First Aid and all of the buttons are greyed out. Impossible to use. I checked via Terminal in /Volumes and, alas, the disk is nowhere to be seen.

Here's what I tried:

• change the USB cable;


• tried to list the disk through Terminal: after having switched it on, it takes a good 20 minutes before it starts to spin and then it shows with the command diskutil list;


• downloaded Kext Utility 2.6.1 and ran the program, but it did not change anything;


• tried mounting and force mounting, but it did not work...

I found this page (a bit old though) which explains how to force mount a drive and here's what I did:

mkdir /Volumes/Backup
sudo mount -t hfs /dev/disk2 /Volumes/Backup

but it gave me a "mount_hfs: Invalid argument"

So I tried

sudo mount -t lfs /dev/disk2 /Volumes/Backup

and it gave me: "mount: exec /System/Library/Filesystems/http://ift.tt/1BIHQ9C for /Volumes/Backup: No such file or directory"

I checked and /Volumes/Backup exists...

I also tried this command (found on this site), but with the same Invalid argument:

sudo mount -t lfs -o rdonly /dev/disk2 /Volumes/Backup/

Finally I tried this (found here):

ps -ax | grep disk2

But it was useless...