Lubuntu 14.04 64 is the only OS on my laptop (HP, UEFI boot). I wanted to have a look at Mate 16.04, so I downloaded the 64 bit ISO file and made a USB live key using Startup Disk Creator with persistence. I tried the live USB and never pressed the button for installing Mate on my HDD. I just found out that persistence did not work. Then I left and was away for one month. On my return (today) I started Lubuntu 14.04 and Software Update warned:
CD/DVD 'Ubuntu-MATE 16.04.1 LTS Xenial Xerus - Release amd64 (20160719)' is required
Please insert the above CD/DVD into the drive '/media/cdrom/' to install software packages from it.
The weird thing is, I never had that CD. I've tried to plug in my usb key with the live version of Mate but this isn't what Software Update requires. I checked Synaptic Package Manager: the list of the repositories now includes Xenial (main, restricted and universe) from Ubuntu MATE 16.04.1 LTS Xenial Xerus. The following packages turn out to have been installed on my system from Xenial Mate main:
dkms fakeroot gcc grub-efi-amd64 grub-efi-amd64-bin grub-efi-amd64-signed libatomic1 libc-dev-bin libc6-dev libfakeroot libitm1 libtsan0 linux-libc-dev manpages-dev
I feel fairly sure that my system is broken. This is the PC I daily use for working, I have tuned it up to meet my needs. I simply cannot afford reinstalling everything now because I don't have time. I'd be VERY grateful if anyone could suggest a way for recovering my system.