How to stop my USB storage from disconnecting when my laptop sleeps

Alas not all USB storage devices respond to this, but there is a checkbox in Apple menu -> System preferences -> Energy Saver for it to prevent from happening. http://www.macsales.com/ can probably sell you a hard drive that doesn't disconnect upon sleep.

You could try & see if by using a card reader and a self-powered USB hub, if the

card would remain visible to the computer in that situation. The power interruption

to the USB bus in the computer may be sufficient enough to affect the card.

While I use card readers on occasion, mostly for camera images when I don't want

to attach the camera to a computer (and don't use native OS X photo application)

these are not left attached when the computer is powered off or let go asleep. I use

them and then unmount the card (by eject menu option or drag icon to trash.)

At some point, if the card becomes invisible to the computer due to improper use

or accidental disconnect without correct unmount procedure, you may have to see

if the Disk Utility may be able to attempt a repair. This may include reformat. But

note, that several cameras or devices that rely on the card to have a specific format

would need to then reformat it again before it would be useful to said device.

And you may need to finally replace the USB storage flash memory or other, should

it continue or get worse in this regard. Some issues represent defective hardware.

Good luck! 🙂

The easiest answer is don't let it sleep; leave it open and set it in System Preferences for the hard drive to never sleep.

You can set the display to sleep without sleeping the computer.