[PLUG] Cached USB Disk Info Weirdness / Fix a USB Disk

[Casey Bralla via plug <plug@lists.phillylinux.org>]

I have recently tried to "repair" a hosed USB Thumb drive, and had lots of trouble. I hope somebody here can help me learn and understand how to deal with what is happening here.

Background:

Using "dd" to create a bootable thumb drive, something goes wrong and then the data structure on the thumb disk is damaged and won't report the correct size.  (It usually shows a few megabytes on a 100+ megabyte thumb drive).  fdisk reports incorrect info about the thumb disk.

Fix Techniques:

Any number of techniques can be used to reset the original data structure.  Some examples: "dd if=/dev/zero...", gdisk "zap disk" command, etc, etc.

Results:

No effect.  The disk structure is still damaged.  None of the documented fix techniques seem to have worked.  Inexplicable!

Resolution:

Reboot computer, and now the thumb drive shows that the fix techniques previously tried have actually worked!  The thumb drive is fixed.

Issue Analysis:

Linux is looking at some type of cached info about the thumb drive and this cached info is cleared during a reboot.  Linux is not looking at the actual hardware of the USB thumb drive.

Questions:

1. How to clear this cached info and force a full re-reading of the USB disk structure without rebooting?
2. Why does the cache even get involved?  Doesn't utilities such as fdisk simply bang the hardware?

Any insight into this would be greatly appreciated!

TIA!

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