Why are my USB drives going offline?

There can be many reasons a USB attached drive goes offline, generally, it is one of the following

1. The drive itself is faulty and is resetting due to a hardware or software fault, or the drive is simply no longer responding in a timely manner due to high error correction (or similar). Drive faults are often obvious as you will hear the drive seeking or the heads resetting (spinning drive only). Also, these are more time-consuming faults, in that you will not see the drive coming back online any time soon.
2. The USB interface has failed and/or performed a reset... or the interface has gone to sleep (check properties, see - https://helpdeskgeek.com/how-to/prevent-windows-from-powering-off-usb-device/). With early implementations of USB 3, we did see many issues with interfaces failing, however, driver improvements have sorted many of these problems. Also, we have seen USB interfaces that simply "reset" under load, this is a hardware issue and little can be done to fix it.
3. The enclosure's USB interface has failed and/or performed a reset. This could be the result of power supply issue (i.e. too many drives spinning up at the same time) or a poor USB implementation.

What can you do?

• Make sure you have the latest USB and/or chipset drivers installed.
• Make sure the interfaces are not going to sleep.
• Often a machine will have a number of USB interfaces, make sure you are connecting to a free interface.
• If possible (for the sake of testing), lower the number of drives in an enclosure to see if the issue improves.
• Add drive letters to the drives to see if the issue improves. We have seen this help with Windows restoring the connection to a USB drive.
• Record the times the drives go offline, there may be some other system issue that is causing this, and the timing could be important.