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Solid State Drives – the true story about SSD performance.

Replacing an old hard disk drive (HDD) with a new Solid State Drive (SSD) will generally transform the speed of your computer, applications open so much faster and everything seems more responsive, however there are still many scenarios where HDDs will actually outperform SSDs …and not by a small margin! Just look below at the write speed of an iMac internal SSD with TRIM enabled – 6.4MB/s.


This is a shockingly slow result for a Crucial SSD that is advertised to have a transfer rate above 500MB/s – why is it so poor? The sole reason for such incredible loss of performance is that just before the test, I used the drive to copy over 50GB of data. That doesn’t sound like a lot on a 500GB SSD but 50GB is about the limit for most SSDs before their performance plummets. After transferring a lot of data they need time to recover and do internal house keeping and garbage collection. Until this is completed the drive may well perform like an old flash drive in a USB-2 keyboard port, while a spinning hard drive will consistently give a reasonable 50-100MB/s.

I see many blogs about how Time Machine has serious problems, or SandDisk SSDs overheat and slow down, but in truth only a small percentage of memory in any sub £400 SSD is fast DRAM – most of the storage is very slow memory. Crucial say the drive needs to run garbage collection and everything will be better again…

“On a Mac®, hold down the Options key while powering on to enter the Startup Manager screen. Leaving the Mac on that screen provides the SSD with power but keeps it in an idle state so Garbage Collection can function, just like the BIOS or UEFI on a Windows® laptop.”

…but if you are using the SSD to e.g run daily backups of a large Final Cut project, you’ll soon see performance slow to a crawl again. In which case you’re probably better off using a cheap hard drive—or better still RAID-0 disks.


The  benefit of using SSDs is for much faster random access, this is what makes an SSD based system feel so much more responsive.


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