Most people are not even aware of the practice of leaving unallocated space on their NVMEs or other types of SSDs. If your “data drive” has 71% free after years of usage this practice would have been unlikely to have made any difference at all. And depending on the design and how recent its design is the practice may or may not make any difference in your long-term performance or the longevity of your drive. But go ahead and leave whatever portion of your apparently new drive unallocated if it makes you feel better... this will not hurt anything.
You might want to install the free version of CrystalDiskInfo available on the Microsoft store on your computer and check the health of your drives. Look at Wear Leveling Count, Percentage Used, and Total Host Writes.
If the wear percentage is under ~80%, your drive is still in great shape.
I am curious which recent Windows 10 update that you believe caused your difficulty. It is possible that something else may have been responsible... Your primary drive on that machine or other hardware may have just failed for any number of reasons... data corruption, malware, mechanical or physical damage from a voltage spike or who knows what else. A windows update gone bad can trigger other issues that were ticking away in the background.
I try to keep a fairly recent image of my primary drive on an external, network or secondary drive available in case of the type of disaster you have described. I always have a couple USB drives close by with Hiren’s Boot CD installed. These are used to get into the computer when it will not boot from the primary drive.
Hiren’s Boot CD (basically no one uses it on a CD anymore) has got a bunch of utilities that can be used to restore an appropriate image back to your primary drive when something goes wrong and also utilities to save recent data residing on your drive before you do this. I use the free version of Macrium Reflect that is on Hiren’s Boot CD for most of my image creation and restore needs. It allows you to do regular incremental backups to whatever media that you want to.
I have a mini-PC running OpenMediaVault (which is free) hooked to an inexpensive RAID enclosure which provides network storage where I save backups of all my disk images. It is not complicated But I am sure that this would be overkill for you. I have a bunch of computers hooked to my home network and am constantly experimenting with various operating systems in both multi-boot configurations and in virtual machines. Most of my failures are caused by me and not the hardware, software or the OS that I am using. There is always something else that I want to try out... mostly because I enjoy technical challenges even in my advancing years.
It is a Linux app that runs from a bootable flash drive. All you have to do is choose the source and destination drives for the backup, and it does the rest.
I’m able to boot the Win 10 Pro machine off a backup SSD (that has an older version of the OS on it): Then I can look at the original (boot) C: Drive. It comes back as healthy, no errors, etc.
Using my laptop, I downloaded from Microsoft a new copy of Win 10 Pro onto a flash drive (ie., I created a new bootable flash drive): The Win 10P machine fails to boot off that, and just goes to the Dell Repair software, which gets me nowhere but “circles”, except it DOES allow me to reboot again off that backup drive. Then the machine boots / runs “normally”. But, I have to keep the machine offline running the older OS release, as “online” it wants to update to the latest and... fatal to it, OS package.
2+(?) years ago I tried downloading CrystalDiskInfo from somewhere - I don’t recall, one of the usually reputable sites, but is wasn’t Microsoft, I’m sure, and someone had inserted a virus — luckily the anti-malware software caught it. I don’t recall the filename, but, I web searched it and is was a real virus, not some problem the anti-virus found with CrystalDiskInfo. At the time I just said “screw it”, as other tests (I barely recall) indicated the drives were ok.
The external drive is HDD, it’s just slow and a bit noisy. I prolly should spring for a new one that might(?) benefit from a USB-C 3.1 connection. I’m not sure the present external backup HDD would...
You may be right that I’m overly concerned about leaving some drive space unallocated. After all, where is 30+ GB of space going already, anyway? Plus, it’s not like I’m suddenly going to toss more data on it in 100 GB chunks.
Thanks for the detailed reply!