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My iMac is very slow and I would like to start from a fresh Mavericks install. But instead of spending the time to backup my internal drive, I would rather just get a big 256GB thumb drive and install Mavericks on that. I would also benefit from the speed of having the OS on a flash drive.

Does anyone see any issues with a solution like this? Since I would be bypassing the internal drive completely as the boot drive, and making the internal drive just another extra HD?

  • it would be terribly slow compared to SSD. – Ruskes Jun 3 '14 at 22:26
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    That is a lot of answers without an up vote ;) – dwightk Jun 4 '14 at 0:43
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Aside from the speed issues that other people have mentioned, you will quickly kill your thumb drive.

One thing that differentiates thumb drives from SSDs is the sophistication of their wear leveling algorithms, the amount of over provisioning of sectors, and the availability to integrate their garbage collection with the host through TRIM commands.

Basically, thumb drives cannot tolerate as many read/write cycles as an SSD. They have only basic wear leveling and very few or no spare sectors if a block dies. In contrast, a SSD can have 10-20% of its total flash capacity just dedicated to wear leveling and failed sector replacement.

As a result, SSDs can handle over 100,000 read/write cycles while cheaper flash thumb drives can handle only on the order of 1000s.

OSes designed to be run from thumb drives (like some variants of Linux and freebsd) try to avoid killing their host thumb drives by mounting themselves read only and using a RAM disk as the swap partition and system log partition.

OSX unfortunately does not have this feature.

In the end, you'll have a slow system that will thrash your poor thumb drive to an early death.

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The main issue I would see is the speed of the USB stick as well as the speed of your USB connection.

SATA is faster than USB. Way more than USB2, a bit more than USB3.

So in comparaison to an SSD it will be way slower.

Just so you get some values here is a little video comparing USB2/USB3/SATA/Thunderbolt

It can be summarised as following : USB2 < USB3 < Thunderbolt < SATA

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I would recommend a speed test on the thumb drive. One simple way of doing this is using Blackmagic Disk Speed Test (App Store). If it's below ~70MB/s speed either way, I'd recommend against it if you're looking for a speedier overall system.

In general, OS X runs just fine on an external drive, but your drive's speed makes a huge difference. In my workplace, we have OS X on an external USB 3 drive (5400rpm) for troubleshooting purposes and it's achingly slow. I've got a recovery partition installed on a small USB 3 stick attached to my keychain and the poor thing gets 10MB/s read/write speeds. Doesn't stand a chance.

But to answer your question, no, there would be no issue other than speed. This functions perfectly fine.

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No problem other than speed.

The SSD has R/W of 400 MBs.

The USB has R/W of <30 MBs for USB 2, and about 70MBs for USB 3.

USB can newer be as fast as SSD since it uses serial communication while SSD use SATA cable connections.

Consequently it is Not very practical to use USB as Operating system disk.

  • Your fact about USB is only true when talking about USB2. – Matthieu Riegler Jun 3 '14 at 22:35
  • > USB can newer be as fast as SSD since it uses serial communication True. > while SSD use parallel. False. That's PATA (the old ribbon-cable drives). All internal buses use serial communication as there are practical limits to parallel buses. Only the main memory to CPU connection is parallel, and it has very strict design limits. The design speed of the bus is the deciding factor in this situation. USB2 plus a basic thumb drive will be much slower than an internal SSD - the storage isn't designed for the same throughput. Perfect for a backup / maintenance device but not for daily use. – paul Jun 4 '14 at 2:19
  • So, if I got a FW800 SSD to use as my OS disk, do you think I would see any speed increases? – Nic Hubbard Jun 4 '14 at 20:32
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I booted Linux with a USB2 flash drive at 20Mb/s read speed. The boot time kills the hard drive boot time, even though the read speed is about 1/4 of the speed. This is a result of better latency speed (no time waiting for the head to position itself over the sector). A flash drive is much slower at reading large files, but much faster at reading hundreds of small files while booting.

Hope this helps!

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