My 2nd internal HDD does not mount

I have a Macmini 5,2, Intel Core i7, 2,7 GHz, 16 Gb RAM. Its boot volume is a 240 GB Solid State SATA Drive (APFS format). I have a secondary HDD, a 1 TB TOSHIBA MQ01ABD100 (Journaled HFS+ format), which only contains data. OS is High Sierra 10.13.6.

Everything worked ok until last week, when I started to get the Toshiba suddenly ejected (mostly after energy saver sent the computer to sleep) ang getting the error message of "disks being improperly ejected". I changed the energy saver parameters to deny the hard disks to sleep.

I stopped getting that, but now the HDD seldom mounts at boot. On these ocasions, Disk Utility doesn't show the disk, so I can't manually mount it. After I run Onyx and the machine restarts, the disk mounts.

I really don't know where to look. Any ideas?