[SOLVED] Please help before I erase again!

[Midnightpainter]

As briefly as possible ... 320 GB MacMini with El Capitan wouldn't log in, then wouldn't boot. Went to Disk Utility in Recovery Mode, volume wouldn't mount, verify or repair. Hard drive itself is verified and S.M.A.R.T. Bought external hard drive, loaded El Cap onto it and transferred files through Recovery from bad volume to external drive. Erased volume, clean install of OS, used Migration Assistant to transfer data and apps back. Everything was fine, everything was there and worked ... until I rebooted. Now I'm right back where I started. HD unmounted and unrepairable. I'm at a loss as to what to do next. Never had a Mac crash before! Any advice would be greatly appreciated.

 

[McHenryB]

Sounds like a bad hard disk or controller. The latter means a new motherboard.

 

[JustSomeJoe]

Depending on age, use etc. it could also be weak/failing Power Supply (rare but does occasionaly happen), or a build-up of dust causing cooling problems, or just good-old bad RAM can cause Disk Problems.

 

[Midnightpainter]

Depending on age, use etc. it could also be weak/failing Power Supply (rare but does occasionaly happen), or a build-up of dust causing cooling problems, or just good-old bad RAM can cause Disk Problems.

Thanks for your response! I've made some progress. Just in case it helps someone else ... I did a triple pass erase, repartitioned the hard drive, vacuumed the vents at the back of the machine and reloaded OS. Disk repair verified volume between every step. Had login issues again, so turned off Cloud and set to automatic login. Mac ran great last night. Today I'll reinstall users and files manually, checking and scanning as I go. Fingers crossed!