I have just been experimenting with Grub launching various Linux distros from a USB hard disk which has FreeBSD 11.0 installed along with a FAT32 partition containing numerous Linux ISOs. All of a sudden I get this UNEXPECTED INCONSISTENCY when trying to boot FreeBSD.

Running fsck_ffs -y -B I get:-

I have no idea where to look.

I was able to mount this partition manually from an alternative installation of FreeBSD but foolishly added an extra mount line in the fstab of that installation to automatically mount '/dev/da0s1a' so now am unable to boot the alternative!
SEE fsck_ffs(8). That means you should type man fsck_ffs. Then lookup the what is written unter option -b. This you can achieve by scrolling down or using a decent search function. At this location start reading.

Yes I can read, and if I understood what this meant I would not have posted a question on the forum...
-b Use the block specified immediately after the flag as the super
block for the file system. An alternate super block is usually
located at block 32 for UFS1, and block 160 for UFS2.

I have absolutely no idea what block 32 for UFS1 or block 160 for UFS2 means. In fact, I don't know what UFS1 or UFS2 means. Do I need to know?
We try even hard to give answers here on the FreeBSD forums. While answering is consuming lifetime, some might think it is less costly to just post a question. Those who are capable to give sufficient answers have worked through what is called learning curve. This is what some managed to have avoided when asking questions here in a frequency that is well above the average. And yes, the quality of questions matters as like the answers given.

So here we go: "Do I need to know?"

This is a matter of mindset and personality. You are completely free to decide in either direction. Depending which way you choose think about the consequences. You may end up bothering others again and again on a subject that should be known, because it might be considered as basic knowledge.

Now to the knowledge about UFS1/UFS2.

I could give you an answer to that or a quote. But an "Aspiring Daemon" with 794 messages here almost for sure stumbled on the term UFS here, isn't it?

So a more appropriate answer here is pointing to Wikipedia. There you can enter as well as UFS1 and UFS2 and the top search results in both cases are sufficient. I've tested this for you and you may try it on your own. Of course you still can prefer not to.

I have installed FreeBSD 11.0-RELEASE. It knows which filesystem has been installed, whether it is UFS1 or UFS2. And as for an "Aspiring Daemon", that is not a classification of my choosing. I still regard myself as a newbie.

Also, I do not regarding posing questions as 'bothering others again and again'. This is an open forum and everyone is free to reply or ignore anyone else. I am not aware of 'house rules' preventing people asking questions. And answers such as 'See FreeBSD Handbook' are not always that helpful when the Handbook is so huge, or check the manpage for a given command when it can have over 100 options many of which can be obscure.

As for the original question about missing superblocks, I haven't found any posts or explanation of what the problem is with my filesystem.
UFS2 has been the default filesystem since FreeBSD 5.0. Only systems older than FreeBSD 5 will have UFS1.
The best search function for The Handbook is Google :)

As for a cmd with lots of options, many of which seem obscure if you are not familiar with it, there's gpart()
gpart(8) does that manpage have "over 100 options"? How did you count?
Now regarding Gpart if you call "many of them obscure" which are the ones? Please be precise!

The problem with many man pages is that they are written by people who understand them for people who understand them. Those people find it difficult to put themselves into the position of someone who is completely in the dark about how to do something or how to get round a problem...

If, for instance, I want to configure a new disk to use gpt, freebsd, ext2fs, fat32 and efi, the man pages for gpart do not explain how to do this. So, suggesting to someone to simply check gpart(8)() is not exactly all that helpful.
No, but it is a starting point. Read that man page. Read the wiki page for how GPT works. Find some web page which describes (U)EFI, and how it interacts with booting and GPTs. And so on. Once you understand all of those things, the man page for gpart will be completely clear and logical.
When I said "I have no idea where to look" it was in the context of this problem. I did check all the usual places, and as you can see no solution emerged and I ended up reformatting the partition which flagged the error. Not ideal.

As far as I'm concerned, ignorance is bliss. If things 'just works automagically' then I'm very pleased about it. If I turn on a switch and a light comes on, I don't believe I need to know how electricity is generated. Similarly, if the operating system knows something, then I don't need to know.

Believe me, I have learned a lot over the last couple of years since I've been using FreeBSD, and a lot of that knowledge has come from users of this forum who are happy to share their experience. One of the problems with FreeBSD is that it has been around for a long time and so have many guides and tutorials, some of which were accurate at the time they were written, but FreeBSD keeps evolving and what applied in 2007 may not apply in 2017, but you can't tell from an article whether the advice is still pertinent today.
No, but it is a starting point. Read that man page. Read the wiki page for how GPT works. Find some web page which describes (U)EFI, and how it interacts with booting and GPTs. And so on. Once you understand all of those things, the man page for gpart will be completely clear and logical.

I have read the man page numerous times, but gpart is a very powerful program with many options and most examples of its usage are fairly straightforward. I did think I got the hang of it but have been unable to create a gpt disk with grub on it. Having said that I've just discovered
grub2-efi. I've been using the grub2 pkg up until now, so I'll give this new one a try..