What Was Happening
A Putney SW15 customer brought their iMac 27-inch 2015 to our workshop on Lower Richmond Road. The machine had become very slow to use over the past year — opening applications like Safari, Pages, or Photos was taking 3–5 minutes. The system beach-ball appeared constantly during normal tasks. The customer had tried restarting more frequently and clearing some files, which had not helped.
The machine had been in daily use as the household’s main computer for six years. It had never had professional maintenance. The customer was considering buying a new iMac but wanted to understand whether the existing machine was genuinely past the point of useful life, or whether it could be restored.
Our Diagnosis
We ran a health check covering all major subsystems.
Storage: The 2015 iMac 27-inch ships with Apple’s Fusion Drive — a combination of a conventional 1TB mechanical hard drive and a 24GB SSD blade that macOS manages as a single logical volume, using the SSD for frequently accessed data. We assessed both components. The SSD blade was healthy. The mechanical hard drive showed SMART reallocated sector counts and an increasing raw read error rate — both indicators of physical degradation. At the rate of increase, the drive had weeks to a few months before failure was likely.
Memory: 8GB RAM, two sticks. Memory test passed cleanly. Not a contributing factor to the slowness, but we noted that the 27-inch iMac has user-accessible memory slots and can be expanded to 32GB without specialised tools — we mentioned this as an option.
Startup queue: Activity Monitor and Login Items showed 23 startup items — a mix of Apple services (iCloud, Photos, Spotlight indexing — all legitimate), and a large number of third-party applications that had been set to launch at login over years of software installation. Several were from applications that had been deleted without removing their startup entries.
Thermal: Fan and thermal performance were within normal range. The iMac’s cooling system was approximately 30% blocked with dust accumulation — worth cleaning but not causing thermal throttling.
How We Fixed It
We discussed the findings with the customer in person (they had walked in). The customer approved the following work.
Drive replacement: The Fusion Drive’s mechanical component was replaced with a 1TB SSD. The original 1TB mechanical drive was imaged before removal. The image was verified and transferred to the new SSD. The existing 24GB SSD blade was removed from the Fusion Drive configuration — with a full 1TB SSD in its place, the Fusion Drive configuration was no longer needed and the 24GB blade was left as a dormant component (removing it from the Fusion volume avoids the complexity of managing a two-drive logical volume when only one is needed).
Startup rationalisation: The startup queue was reduced from 23 items to 9 — retaining iCloud Drive, Photos, Spotlight, and a handful of applications the customer confirmed they used regularly. Eleven items were removed, five of which were orphaned registry entries from deleted applications.
Dust removal: The cooling system was cleaned via the machine’s internal vents to the extent possible without full disassembly.
The Result
Application launch times reduced from 3–5 minutes to under 10 seconds. The machine returned to the responsive performance it had when new. The beach-ball cursor, which had been near-constant, did not appear once during our post-work testing.
The customer collected the machine the following morning and, by their account, was surprised by how significant the difference was. The iMac was back to being the household’s main computer rather than a source of daily frustration.
Why This Works — iMac Fusion Drives and SSDs
Apple’s Fusion Drive was a practical compromise in the years when large SSDs were expensive. The logic was that most users access the same applications and files repeatedly, so a small fast SSD acting as a cache for a large slow hard drive would produce SSD-like performance for typical use. In practice, the system works well for light, repetitive use but degrades as the mechanical drive ages and the machine’s usage patterns diversify.
Replacing the mechanical component with a full SSD removes the Fusion Drive compromise entirely. The machine behaves as if it has always had an SSD, because all storage is now SSD. The 2015 iMac supports SATA-III SSD speeds which, while not as fast as NVMe (which requires newer hardware), are still 5–7 times faster than the mechanical drive they replace.
For an iMac that is otherwise healthy — good memory, clean thermals, functional display — this is one of the highest-value upgrades available and routinely extends the machine’s practical life by several years.
Prevention Tips
- If your iMac has a Fusion Drive and is more than five years old, a SMART check on the mechanical component is worthwhile — they tend to fail in a predictable pattern after extended use
- Review Login Items in System Settings once a year and remove anything you don’t recognise or use
- A 27-inch iMac can be brought to our Putney workshop directly — or we can collect it. Don’t carry a 27-inch machine on public transport
Local Help in Putney SW15
We carry out iMac and Mac health checks at our Putney workshop on Lower Richmond Road. Walk-ins welcome for drop-off — or call to arrange collection if the machine is large.
Related Services
- PC & Mac Health Check — full diagnostic service for any Mac or PC
- PC & Mac Health Check in Putney — walk-in or drop-off at our Putney shop
More Case Studies
- iMac hard drive reinstall in Putney — another iMac storage repair at our Putney workshop
- PC & Mac health check — slow MacBook in Wandsworth — health check on a MacBook with overlapping causes
- MacBook Air hard drive replacement in Streatham SW16 — SSD upgrade restoring a slow MacBook to reliable performance