Those checkmarks are not for diagnostic purposes. Blackmagic is a company that produces medium to high end video production equipment for studios, and Blackmagic Disk Speed Test is intended for professionals that want to determine whether their internal disk is fast enough to record a type of video without frame drops or performance issues. So a MacBook Pro with a standard hard drive (non-SSD) will always have grey or red "X"s on most of the boxes, because the drives in those machines simply aren't fast enough even when they are working perfectly.
That said the
Hard Drive Cable in the MacBook Pro (13-inch Mid 2012) is notoriously unreliable and can cause all kinds of performance issues. Sometimes it works fine, sometimes it slows down, sometimes the drive won't mount or detect, sometimes the computer won't boot. As a matter of fact Apple has an active quality program for defective drive cables. I replace about 10-20 a week, and for some that would be considered low volume. It's extremely common.
The drive could also be failing. Not unheard of by any means.
Knowing these machines, I typically run SMART Utility over the drives. (
https://www.volitans-software.com/apps/smart-utility/ ) If SMART Utility shows "PASSED" next to the drive name, then there is a good chance the drive is fine, and I replace the hard drive cable. If it shows "FAILING" or "FAILED", then I replace both the drive and the drive cable at once to resolve the issue.
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