When your Mac’s hard drive starts to fail, a lot of unexpected things can happen. First, your Mac can erratically restart from time to time, without warning. This usually happens when Mac OS is actively looking for an important system file and it hits a bad cluster causing it to panic restart itself.
Another common symptom of a bad HDD is when your Mac freezes up, when you are doing simple things like checking your email.
So, it is very important that you check to see if your Mac’s disks are in good health. Go to Disk Utility and scan your Mac’s hard disks. It is recommended that you scan all hard disks but your Mac OS partition disk is the most important disk to scan for health.
If there are no errors reported, please check for free disk space. Lack of free disk space on your Mac OS partition is one of the primary reasons why your Mac runs suddenly slow. Create free space by either deleting unwanted files or folders or my moving them away to external hard drives. When moving files, it is important to cut and paste and not copy and paste files as copying will leave a copy of your files on your OS partition. Of course, copying and pasting and then deleting the old copy is the best way to copy files to another location as you can have a backup copy in case you lose power or see your Mac restart abruptly, whilst the copying is taking place.
You can also run Apple’s hardware diagnostic test to see if your Mac’s hard disks are doing OK. To do this, you will have to restart your Mac. Since initiating the Apple hardware diagnostic test is different for each Mac model, we recommend that you look up the correct instructions at Apple’s official support site.