Since dedicated mining OS can properly run on a single USB, miners usually go that road and pick up first UBS drive they find in a drawer. While majority of miners don't have any issues with running (any) mining OS on a USB, it is still more recommended to run a dedicated mining OS on an SSD.
Even if you are running a 32 GB USB 3.2, not all USB drives have the same response speeds and characteristics. Some are fast and OK, but most of them are slow.
While this doesn't effect the mining process itself, it effects the following aspects of your mining rig:
Every second lost is the second your rig is not mining.
For comparison: A rig running on a 16 GB USB 2.0 can take up to 3 minutes to properly boot, a rig running on 32 GB USB 3.2 can take up to 1 minute to properly boot, and a rig running on an SSD can take few seconds to boot.
USB drives are not the best performing when it comes to constant writing, reading, and erasing. If you have mining client logs turned on, it can quickly get corrupted and reflashing is needed. With SSD you don't see such issues.
USB drivers get hot pretty fast and it is not good running them at high temperatures all the time the rig is running.
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