iMac Repair8 September 20265 min read

iMac Taking Minutes To Start Up? The Drive Is Usually Why

An iMac that takes minutes to boot and beachballs on every click is rarely short of power. It is usually still running on its original spinning drive.

Why a spinning drive makes a good iMac feel broken

Intel iMacs were sold for years with a 5400rpm laptop hard drive inside them, or a Fusion Drive that pairs a small SSD with the same slow disk. That was tolerable when the machine was new. It is not tolerable now: modern macOS writes to the disk constantly, and on a spinning drive that turns into a machine which takes two or three minutes to reach the desktop and beachballs every time you open Mail. The processor in a 2015 or 2017 iMac is still perfectly capable. The drive is what is holding it up.

A drive that is slowing down is often a drive that is failing, and the two feel identical from the outside. Bad sectors make macOS retry reads over and over, which reads as sluggishness long before anything says the word "error". If it is a Fusion Drive, the SSD half can fail on its own and leave the whole volume limping. Either way, the window where your data can be copied off cleanly is while the machine is slow — not after it stops booting.

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Open Activity Monitor and watch the Disk tab while it is being slow. High disk activity with low CPU points at the drive, not the processor.
Check About This Mac → Storage. Under about 10% free will slow any Mac down on its own, regardless of the drive.
Notice when the beachball appears. On opening apps and switching windows means storage; only inside one heavy app means something else.
Put your ear near the machine in a quiet room. Constant chattering is a hard disk working hard; rhythmic clicking means stop and back up now.

Telling a dying drive from a full one

Slow and failing look the same to the person using the machine, so the first job is to separate them. Most of this we can do at your desk; the SSD fit itself usually means taking the iMac away, because opening the display without damaging it is bench work.

Read the drive’s SMART data for reallocated and pending sectors — the difference between "slow" and "dying" is written there.
Establish whether it is a plain hard disk or a Fusion Drive, and if so which half has gone, because that changes the repair entirely.
Confirm storage really is the bottleneck before quoting an upgrade, so you are not paying for an SSD that fixes nothing.
Clone the existing system onto the new SSD rather than reinstalling, so your apps, files and settings come across exactly as they were.

Keeping a new SSD quick

An SSD will not slow down the way the old disk did, but a few habits keep it quick and keep your data safe.

Keep at least 15% of the drive free. macOS needs room to work and gets slow long before it gets full.
Get a Time Machine backup running now rather than once the machine is misbehaving.
Treat the first unexplained beachballs as information. They are usually the earliest warning a drive gives.