My attempts to back-up to an external hard disk finally met with apparent success but I cannot now mount the target drive.
To summarize:-
1. Installing a SATA hard disk, identical to my computer's main drive, in a USB 2.0 caddy and attaching this to the computer resulted in qualified recognition. 'fdisk' 'saw' both the main drive, as sda, and the USB drive, as sdb, respectively but initially noted that the latter, “... doesn't contain a valid partition table.” This was hardly surprising. At the point of first connection the external drive was essentially a 'bare metal' device, having had its data wiped. Nevertheless, 'fdisk' correctly reported its size, number of heads and cylinders, etc.
2. 'dmesg' also correctly identified the external disk as sdb, reporting its type and the USB port to which it is connected.
3. 'df' ignores the second hard drive, reporting only the main disk.
4. Attempts to clone/back-up the main disk to the external disk using the recommended tools EaseUS Todo back-up and RedoBackup, booting respectively from appropriate USB memory sticks, both failed. Neither utility was prepared to write to the external disk. EaseUS acknowledged the latter but crashed the whole computer when instructed to perform the clone. RedoBackup failed to recognize the external disk.
5. Then came the break-through. A bit more Net browsing led me to try the command 'dd if=/dev/sda of=/dev/sdb'. The main disk was bit-copied (cloned) to the external disk at roughly 20Gb per hour, meaning that my 80Gb disk was copied in just under 4 hours. Checking 'dd's resulting report showed what appeared to be a perfect copy.
So far, so good. I now have a back-up which, being identical to the main disk, should, I assume, be bootable. In the event of trouble with the main disk I should be able simply to exchange it for the external disk and carry on from the point at which I made my last back-up. I cannot, however, access and read the external disk. I assume that it must have a partition table and be mounted. The first requirement appears to have been resolved by the cloning operation. 'fdisk' reports no trouble with the partition table on the cloned external disk. It lists /dev/sb1 as the bootable Linux partition, /dev/sdb2 as the Extended partition and /dev/sdb5 as the Linux swap / Solaris partition, exactly mirroring the corresponding entries for the main, sda, drive.
My attempts to mount the external disk have all failed, however. I clearly do not understand the syntax of the mount command or have failed to meet some other requirement. 'mount' objected without hesitation to my early mistakes but “mount -t dev/sdb” was instantly accepted. No error messages or other output resulted and the command prompt was immediately redisplayed but 'mount' then failed to show that the external drive had been mounted. If I try something like 'mount -t /dev/sdb1 /mnt/xdisk', where xdisk is a directory I have been told to create, then I am presented with a prolix description of 'mount's syntax, most of which leaves me bewildered. 'mount' then once again confirms that sdb1 does not feature in the list of mounted devices.
Can someone offer any suggestions? I have read one or two of the other posts on this topic but none of the details match my problem too well and I did not understand some of the replies. If I try to add a line to /etc/fstab, for example, I find that I do not have a directory called 'fstab', only 'fstab.d' and that is empty.
I have roughly one year's experience with Linux Mint 13 which I chose because it so closely resembled Windows XP.