Chosen Solution

hi @mayer! iʻm on a previously used/refurbished mbp 9,1, 16GB, 1TB SSD, so definitely third party additions. Iʻm currently running Sierra 10.12.6, but now i am falling off backward compatibility for various software that i need for work.

  1. is upgrading to 10.13.6 likely to give me problems as the OP experienced,
  2. likewise for 10.14 or 10.15, in general, will i be ok on this machine upgrading to new OS? i see refurbished 9,1ʻs sold on the market with 10.14, but donʻt know if they have problems. thanks so much!

Your 2012 15" MacBook Pro should stay at Sierra (10.12.6) as being the best due to the differences between the older HFS+ and newer APFS file systems. The older HHFS+ runs better on SATA drives. If you must, I would go to Mojave (10.14.6) ands no farther or High Sierra (APFS beta). Catalina won’t run older 16/32bit apps and add-on’s. You need 64-bit apps/add-ons so make sure you don’t have any dependancies on the older 16/32bit. Catalina (10.15.x) is as high as you can go with this system. But your biggest issue is the HD SATA cable! Your Hard Drive Cable Is A Ticking Time Bomb I strongly recommend replacing it and applying the tape as well. MacBook Pro 15" Unibody (Mid 2012) Hard Drive Cable & MacBook Pro 15" Unibody Mid 2012 Hard Drive/IR Sensor Cable Replacement