It's physically an IDE HDD? (Ribbon, not sata connection)
By rights setting the BIOS to AHCI should have made that drive invisible from the get go
But if everything was working fine (same config, same BIOS settings and RST drivers) and it suddenly started playing up - it could be the SSD itself.
But if the HDD is 'going to sleep' while in use - that drive itself could be failing.