Have you tried booting from usb using plopbt? You put plpbt.bin put in the boot directory point grub to it then plopbt gives the option to boot from usb. That is what the plpbt is for, to be able to boot from usb on older hardware that doesn't normally allow booting from usb.

The point colorpurple21859 is making is that the PLOP CD is capable of booting a iso on a usb. That works on computers that do not boot from a USB. You boot the CD, and PLOP does the rest.
The Rescatux/SuperGrub2 CD works the same way (only better, IMHO)
Rescatux tutorial - http://community.linuxmint.com/tutorial/view/1850

Since colorpurple21859's link did not work, here is corrected link -
http://www.plop.at/en/bootmanager/plpbt.bin.html

PLOP only works on IDE connected CD drives, doesn't work for SATA drives. Rescatux/SuperGrub2 works on either IDE or SATA.
http://www.plop.at/en/bootmanager/faq.html

EDIT: I see colorpurple21859 made a new post while I was composing this post...

You can't make permanent changes to a CD/DVD but while you are booted to it you can make changes. Any change you make will be lost on reboot. In the case I suggested it wouldn't be a problem. I found a Mint Mate DVD and tried editing the /etc/mtab file and the change was saved. I still got the warning though. I know this worked on an earlier version of Mint, Mint 13 I believe.

Anyhow, I booted the Mint DVD and started the installed and selected a partition on which to install, selected ext4 format, / mount point and got the message about writing changes to disk. I clicked the Go Back tab, and selected the partition, format and mount point and clicked the Continue tab and let the installer run. It finished successfully so if that doesn't work for you, there is some other problem.

TxLonghorn: Unfortunately, it doesn't seem so. I made the changes you specified (I am in fact using an iso file for XUbuntu12.04, not Mint 13 xfce which is unpacked on the DVD, but I changed the iso file name to reflect that). What happens is the same as last time: it loads fine as Live boot; I click "Install", I set up the install parameters to install to sda1, with format, set as root, etc; it gives me a warning message and, as you instructed, this time I tell it to unmount as necessary; it then gives me the slideshow that you get when installing Ubuntu-derived distros; I start telling it my location on that map thing, testing the keyboard and telling it my log-in name and password etc. Then it pops up another warning saying it cannot install because /isodevice is still mounted and it needs to unmount. You only get Continue or Go Back, so I continue. The installer then has, at the bottom of the window it's playing the slideshow on, the message "Detecting file systems...".

And there it stays. No flashing of the HDD light, no movement on the progress bar - but the slideshow continues to run, and I can even access things on the live boot. In fact, I am typing this as the installer is (not) running in the background, by loading up Firefox from the live boot. I opened up a terminal and did "mount", and it still says, amongst all the other stuff:

So I've told it to unmount /isodevice, but that's what it's installing from, so I imagine it can go no further.

It's not that it is actually installing, and I'm being impatient. In the time it's taken me to type this post, it's been going along in the background. Still "detecting file systems".

I'm going to post this message and close down XUbuntu using its normal menu options. At least it hasn't crashed...

If I was going to follow the directions of other contributors to this thread, about editing certain files on the live boot to comment on references to isodevice, how does that affect the amendments to grub.cfg you already proposed?

Thank you again for your assistance in this matter.

I am sorry that didn't work. I followed that exact procedure I gave you, with the Mint 13 iso file on sda2, and installed Mint 13 to sda3 successfully.
Maybe it would work with the iso file on a flash drive instead of the hard drive.

I can't think of much more to try. I do know of a couple of cases where people faced similar problems, and pulled the hard drive out and mounted it in a different computer, then installed Mint to the hard drive, and moved it back to the original computer. That works, as long as no special drivers have to be installed for the graphics.
The two are completely separate. Editing the files in the live environment will not change anything on the hard drive.

Ah! Success!!

Please play this video for a few seconds...

Yep, that's how I feel.

With the amendments to the grub file you outlined earlier, I followed Yancek's advice and edited the file /etc/mtab as instructed, once the live Xubuntu from the iso on sda5 had loaded up (open the /etc/mtab file and look for the entry with '/isodevice' and comment it out by putting a hash mark at the beginning of that line). Had to do it as root.

With that amendment, I still got the message about mounted partitions on sda, but did not get the one about /isodevice this time, and the install started proper!

Upon restarting, my old Linux 12 LXDE did not appear in Grub; however when I ran the usual copious updates, it ran update-grub during those and it appeared on the grub menu after that.

So, now the updated distro is now installed on sda1, I can focus on copying over relevant bookmarks and config files from my old distro on sda5 and the usual "bedding in" procedures for a new install.

Well, a big thank you to all the contributors on this thread; with honorable mentions for TxLonghorn and Yancek. I literally could not have done it without you.

YaaaHoooooo !
That was a tough nut to crack. Congratulations.
It will feel good to mark this thread as SOLVED, right?

It certainly will. I assume the same technique of editing /etc/mtab would have worked on the DVD version of Mint 13 xfce as well; however I've got XUbuntu 12.04 installed now, so a change is as good as a rest. And if I get the same problem on future installs on this laptop, I'll now know how to fix it.

Hopefully this thread will help other people who may be having the same problem with an install.

I also learned how to boot an iso direct from a hard drive without having to unpack it to a DVD or memory stick, so that's a bonus.

Thanks again.

This has been a problem with Mint for several releases, at least for some of us. Editing the /etc/mtab file is something I actually found on the Mint forums several versions back. I usually have a partition created before hand on which to install, so when I get the message about partitions being mounted and asking if I want the installer to unmount them, I select No because as it states, it only applies if you want to create, delete or format a partition. And yes, editing a system file like /etc/mtab you do need to be root which means using sudo on Mint.

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