How to convert EXTERNAL BOOTING SSD HFS+ with MacOS 10.12 Sierra to APFS-formatted with macOS 10.13 High Sierra?

Only if the HFS+ drive is erased of all files can it be converted using Disk Utility. Otherwise, it can be formatted directly to APFS by Disk Utility.

Prepare for APFS in macOS High Sierra - Apple Support

Pondering the conversion from HFS+ to APFS | Carbon Copy Cloner | Bombich Software

Can APFS be used with Time Machine, Boot Camp, and File Vault?

APFS is better... as in better taste, less filling. I know that's a flippant exaggeration but it's not too far from the truth.

There is a lot more along those lines in this Discussion:

Re: When I installed High Sierra on my external USB 3.0 SSD boot drive, APFS was not automatically applied to re-formatting that drive.

The only compelling reason to convert your external SSD is to satisfy your own curiosity.

But I also understand that for internal High Sierra booting SSD, it is better to format as APFS, and that is why Apple does that automatically without allowing users to do otherwise. Right?

Better or worse doesn't matter. You don't have a choice. The HS installer will convert all-flash storage-equipped Macs to APFS with the emphasis on all-flash. As I wrote don't convert it on your own first, because I don't know what will happen if you do. Go ahead and convert it and find out for yourself. Then, you can be the expert.

All I know is that there have been many reports of problems from those who did, but their underlying causes are many.

But if that is so, my question is: what is the advantage of APFS for internal High Sierra booting SSD that do not have external ones?

APFS has certain undeniable advantages that you can read about for yourself here:

https://developer.apple.com/library/content/documentation/FileManagement/Concept ual/APFS_Guide/Introduction/Introduction….

That's all I can tell you, and the only thing to add to that is that Apple's concern is primarily with devices incorporating built-in all-flash storage. External devices are way, way down their priority list.

I would read the following rather than asking the questions here:

1. Is It Possible Not to Convert to APFS When Upgrading to High Sierra? | Other World Computing Blog
2. How to Skip Converting to APFS When Installing macOS High Sierra
3. Is It Possible Not to Convert to APFS When Upgrading to High Sierra?
4. VP of Software Engineering Dives into APFS
5. Video Presentation- Taking A Deep Dive into Apple's APFS
6. Pondering the conversion from HFS+ to APFS | Carbon Copy Cloner | Bombich Software
7. APFS with Time Machine, Boot Camp & File Vault

No, it's correct as written. I suggest reading the articles linked which should answer any questions about APFS including pros and cons.

APFS is Apple's filesystem of the future but it remains an incomplete system which works reliably only on SSDs. It is not recommended for HDDs or Fusion Drives. Unfortunately, only experienced developers probably understand it. It is quite different from HFS+, so even if you know that well it won't help much with APFS until you get yourself thoroughly educated. I do not qualify as an expert with APFS. My knowledge is superficial.

You have to erase the drive before Disk Utility will give you the option to convert to APFS. Once you erase the drive it won’t be a bootable anything, so the question of whether it used to be a bootable Sierra drive makes no sense.

Note: At this point there is no real advantage in having an external drive formatted as APFS. The one external SSD that I experimented with had slower performance than with HFS+.

I know what he's talking about, Lanny. If it was an HDD there would be no point to my posts. I have been working with APFS since before its release, so I pretty much know what we can do with them when it comes to formatting. Apple has somewhat confused matters by forcing users to convert or reformat their SSDs using APFS without the users' awareness. I appreciate your comments but not really sure what led you to think I had the type of drive confused.

AFAIK, Sierra is not supported on a bootable APFS formatted drive. Sierra will recognize external APFS drives, but High Sierra would be needed otherwise. I don’t know of anyone being able to run Sierra on an APFS formatted drive. You’re welcome to try.

Since you asked, yes it can be done, yes I have done it.

Is it recommended (pros and cons)?

Recommended... by whom? Not Apple, anyway.

Advantages: A big shrug, mostly. APFS is better than HFS+ for a variety of reasons you already know about, but internal vs. external makes no difference to it.

Disadvantages: Apple does not specifically recommend it, nor do they specifically advise against it. Do it for your own edification or entertainment.

You did not explain your reasons for wanting to do it so I'm assuming it's just curiosity, which is the reason I did it. I found no noteworthy performance increases, nor did I find any particular disadvantages. It was boring. After a period of cursory testing I moved on to more important things.

OK. I will make a Time Machine backup of bootable external SSD, format it as APFS, install High Sierra on it ...

Stop there. Back up or otherwise archive the contents of that SSD, but then just install High Sierra on it in the usual manner. The installer will perform the conversion to APFS. After it finishes, then installer will ask if you want to transfer information from another disk, same as it always has. That's the normal, extensively tested and deployed upgrade process.

Converting a Sierra boot volume to APFS and then installing HS is unconventional since the installer will expect to find a Sierra (or earlier) OS on an HFS+ formatted volume... not one already converted to APFS. I don't know what will happen if you perform the conversion first. At the very least I expect it won't boot Sierra any more. As long as you have a recovery plan, why not find out for yourself?

A firmware upgrade is incorporated in the HS upgrade, as was the case with earlier versions of OS X. The installer expects to find Apple-supported hardware. Since Apple does not write firmware for hardware they don't incorporate in the products they build, you may encounter a problem when that firmware upgrade attempts to install on your aftermarket SSD. Those problems may manifest is a failure to boot, or performance problems that may not be immediately evident. Once again, find out for yourself.