I seem to be digging myself deeper with my USB 3.0 Samsung T3 external drive that is running OS Sierra 10.12.6. The T3 has been my boot drive for my 2012 i7 16GB RAM Mac Mini since December 2016. I lost the ability to boot the SSD immediately after installing an Apple security update. When I did an NVRAM reset the SSD did not show up on the available drive list. I was finally able to boot using the SSD but it required reinstalling Sierra in order to accomplish it. I used a bootable external cloned volume. (Previous Thread: https://forums.macrumors.com/thread...tartup-disk-changed-to-another-drive.2113860/) Kindly note that until a couple of days ago the SSD was operating with less than 10% free space, which may have contributed to my problems. I was getting ready to purchase a larger replacement SSD when I installed the security update. After regaining control of the boot SSD, I installed Disk Sensei (https://cindori.org/disksensei/) to avail myself of its various tools. I was able to locate and delete old mobile app files left over from when iTunes managed iOS apps. That alone got the 250GB SSD to regain a total 86GB free space. Previously it had around 19GB. (I will probably be able to reclaim more space as well when I have time to go through the entire list.) I decided to enable "automatic TRIM" using Disk Sensei, thinking that it would help maintain the health of the SSD. After doing that, I noticed a decrease in the WRITE speed to between 34-54MB/s while READ remained around 450MB/s, which is within the normal speed range. WRITE is usually somewhat less fast. I tried to boot into Single User Mode but I was not able to do so. I again tried NVRAM reset with the idea of being able to enter Single User Mode to use fsck -fy to force TRIM and return the SSD to its previous speed but I again lost the ability to boot the SSD. I booted from a cloned external volume with an earlier OS. I reinstalled Sierra a second time. After regaining control of the SSD, I disabled automatic TRIM in Disk Sensei. But the SSD speeds remain in the same range. And that is my Tale of Woe. Gawd Knows I probably did some things wrong out of sheer ignorance. . . I haven't had an experience like this since my OS 7 days. My goal is to get the SSD back to where it was before this all began, including running any diagnostic steps necessary to assess the cause(s) of my problem (besides what I did to myself). I'm asking for advice because I don't want to make things worse. I'm apparently out of my league regarding the SSD issues.