I am constantly encountering the following problem with my Mac(s). This has been happening with two different Macbook Pros (13" late 2011 and 13" 2017) with numerous MacOS versions, including the latest High Sierra. In addition, this happens with various different USB pendrives.
We are talking two different Macbooks here, i.e. the USB controller is hardly the problem. We are also talking about multiple, different USB pendrives, so the drives themselves are also not the problem.
With the new Macbook, I have to use a USB-C to USB-A adapter. Here, I use one of the brand "LPG" and one from Apple. The Problem persists with both adapter as well as with either one of them alone.
Obviously, this is not a problem with a specific system configuration, version, or hardware, but rather seems to be a problem of the OS itself.
I have a USB pendrive attached to a USB port of an external USB hub. The drive works fine, no problems. Then, I put my Mac to sleep and disconnect the Hub. When re-attaching the Hub at a later time, the drive won't be mounted automatically again. Actually, It doesn't even show up in the list of USB devices in the Disk Utility tool.
Interestingly, all other devices attached to the same hub (keyboard, mouse, ...) keep working just fine after reattaching the USB hub. Never had a single problem here.
Unplugging the pendrive and pugging it in again won't help, even when plugging it into another USB port. But when the system is rebooted, the drive shows up again as if nothing happened.
How do I stop this from happening?