Degraded read speed using Mac OS drive formats

Nominal performance using exFAT on a 2018 MB Pro running Catalina 10.15.5. Read speed drops ~60% when using Mac drive formats. I’m using the 1TB Silicon Power Gen 3 x 4 M.2 2280 SSD. I’ve done the Plugable recommended firmware upgrade for this SSD mfg - no effect on this problem.


Thanks for reaching out to us, I am sorry Apple’s native file systems are not performing well on the external enclosure and I am happy to go into details on why these file systems do not perform well on high performance external drives.

Apple’s HFS+ file system was originally designed with spinning hard disk media in mind and when used over USB has always suffered from reduced performance. APFS, the newer file system from Apple, while designed for SSD media has generally worse performance over USB than HFS+. Most likely this is due to poor optimization that can be significaly resolved over time as Apple improves the file system.

Both of Apple’s file system options are journaling file systems, these keep a log of changes written to the drive that can be used to help prevent corruption and restore the state of the drive after an unexpected power loss. For internal drives the journal can be used to delay disk writes until the drive is in a less active state, or to group similar commands to help performance. For USB attached drives the operating system forces the drive to commit changes when received, instead of pending them to the log, to help prevent data loss when unplugging the drive.

External drives lose almost all of the benefits of journaled file systems while also taking a hit to performance from the unoptimized command order and additional writes to the journal.

Non-journaling file systems like exFat are well suited to external drives, without the additional overhead of journaling they can utilize almost the full bandwidth of USB 3.0 10Gbps, some overhead still exists for both device and software communications.

This may limit the performance for some applications that require a journaled file system, for example some virtual machine software requires journaling to be enabled for storing the guest operating system images.

Please let me know if this helps to explain the performance differences between exFat and HFS+ and APFS on external drives.

Plugable Technology


We’re closing this thread due to inactivity, but if you have any further questions please feel free to contact and we’ll be happy to help.


Plugable Technologies