Wed 3rdMay.
Si et al,
Success at last! The news is good.
I had to wait for almost a week before the aforementioned Samsung 250GB T3 external SSD arrived, but at midday I was able to get to work with it.
First on the agenda was to use the Erase tab in Disk Utility to delete the as-delivered partition on the Samsung disk. I overwrote it with a single-pass of zeros just to make sure. It'd come with an onboard security app, suitable for several different platforms - which I didn't need. The zero writes took some 20 mins to complete, which seemed pretty nifty compared to my previous usage of USB stick(s). I then chose the Partition tab in DU and named and made a 40GB Mac OS Extended Journaled GUID partition on it. The other 210GB remains unallocated and could at some later date perhaps be used for other things. This was all done, incidentally, with the Samsung disk plugged into my powered hub. I left all other external disks and peripherals connected.
I then made sure there were no applications still left open on my iMac. Meanwhile, "Install macOS Sierra.app" was sitting in my Mavericks's Application folder and I decided to execute it from there. There was the usual agreeing to the licensing to do, etc., followed by being asked which destination disk for Sierra was to be used, so obviously I selected the Samsung disk, the volume's name showing clearly in the display of usable disks. At that point something called 'Install Assistant' intervened and required me to enter my Mac login password before the install process would proceed any further.
I half expected the overall install process to involve several mini-installs and restarts, and that was exactly how it turned out. The first phase of the install process took just 4 mins. When that finished a new install sequence started, which had a duration of about 15 mins. Quite different to all the slowness and waiting-around that I'd endured with the USB sticks a week or more ago! With the first install phase, I got a little concerned, as one of my other external SSDs also on the hub became dynamically activated for a minute or two and I wondered why that would happen. However, in retrospect the contents of that disk have remained unaltered, so maybe I'd merely witnessed a flurry of control transfers across my powered hub?
A further restart occurred but was quickly replaced by a Welcome screen. This was, of course, where Install Assistant showed itself, and I had to then spend some minutes configuring the keyboard country, wi-fi (not relevant in my case), turning off Siri, and opting out of Share Analytics and Crash Data for Apple. As you might expect, I was also asked for my full name, account name and Mac login password to use on Sierra. I decided to keep to the existing ones I use on Mavericks (after all, this externally-bootable Sierra is just for trying out the operating system, not as anything permanent). I was also asked which of the relevant external disks in my setup I wanted to use for Time Machine backups. I wasn't sure at that point whether TM would, during the next week or two of assessing Sierra, prove useful, but I decided nonetheless to give it the go-ahead. Somehow, it came up with a figure of 98.79GB needing to be backed up! Goodness knows where it got that figure from! Anyway, this 'first full backup' then proceeded in the background, and completed about 40 mins later. I went away and made lunch in the meantime. (In retrospect, it may have been better to have not opted for TM, since this has now subsequently eaten into the TM disk's remaining capacity, causing some deletions of earlier TM images). But anyway, as soon as the TM backup initiated, the screen automatically booted into Sierra, to the default Desktop, with my relevant external disks showing. Like I say, I decided to then let TM do its stuff for the 40 mins, before then me making a start on investigating Sierra.
I've only spent about 5 or 10 mins so far, setting up various things in System Preferences, but things are looking much, much better than a week or two ago, when I managed by the other method to get Sierra on to my iMac as a supposedly 'clean install' but which turned out to be anything but clean. Remember? Switching back to Mavericks was now a doddle, requiring me to simply go into Startup Disk in System Preferences and then logging in using the padlock so as to be able to choose the Mavericks (internal) disk rather than the Samsung Sierra disk.
I must say I've been very pleased with the Samsung disk itself. It features a metal case surround and measures about 3in x 2in x 0.5in, and comes equipped with a USB3.0/3.1 port and a 20in USBa-to-USBc lead (although I bought and used a Startech lead that was somewhat longer than that). The quality of manufacture of the disk appears very good and inspires confidence in it. Its speed performance seems really good. The casing got mildly warm during those fast writes, while installing Sierra on to it, but for something that small you'd expect a compact thermal density, I guess. I've actually raised it off the table surface by applying a set of small stick-on feet on its underside; that should keep a little more air circulating around it; but that's just me being particular. Can't fault its speed performance. In fact, thus far I can't preceive any lagging at all when running Sierra from it.
Clearly, using a half-decent external USB drive has been key in this - which I think is what you contended, Si, quite a while ago. So, thanks for pushing me in the right direction. I'm well pleased. Before, I can't think why on earth I faffed around putting the Sierra Installer on to a USB stick and consequently messing about with a further stick, when all I needed to do was to download the Sierra upgrade file on to my Mavericks machine as normal, and then execute it from there, putting it on to a good-quality external disk.
Whether I'll still be as pleased after a couple of weeks of exercising Sierra using this new booting arrangement will be another matter entirely, though. For one thing, I'll be very disappointed if I now find that there's still no sound notification for the inbox in Apple Mail. But if that's the case that'll all be down to Apple, not a fault in the installation or in the hardware. I've yet to configure my network settings and reinstall my mail account. Incidentally, at one point during the install processes, Assistant stated that it could, if required, get and install all my network and e-mail settings from my existing TM disk. I declined at the time.