Linux agressively caches files in order to improve overall performance.
When copying large amount of files to an USB storage this often results in some wait time until the device can be safely removed.
How long you have to wait depends on your system and the quality of the USB storage device.
Numerous issues regarding this has resulted in various suggestions involving sysctl tweaks and trim.
udev rule to disable write-cache
In another place one user threw in an udev rule which would disable write-cache for devices when they were added and while it was just an idea - it triggered my curiosity.
I dug into the intricacies of udev and found a method to only target USB storage devices.
* only if the device environment ID_USB_TYPE==‘disk’
* run hdparm -W 0 /dev/%K
Create a file in /etc/udev/rules.d/99-usb-sync.rules and paste above content into the file and save it.
Install hdparm package.
sudo pacman -Syu hdparm
Then plug an usb device - open in your file manager - copy a huge amout of files to the device - when the copy is done - click eject in the file manager - and the device ejects immediately.
For those preferring that I have created a PKGBUILD 2 for it which will pull the hdparm dependency upon installation.
