unable to write to external drive after installing el capitan

This typically means that the drive is formatted as NTFS, in which case you need to update the software which previously gave Mac OS X the ability to write to that format.

In the event that this occurs for a FAT32 or exFAT volume, either the volume's directory structure is damaged, or it's locked in hardware, or the drive has sensed that it's about to fail.

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See what I wrote above. The software which enables Mac OS X to wrote to NTFS volumes is a third-party component and needs to be separately updated.

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Go to the software creator's website and look for an updated version.

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I came across this web page and I did this to my microSD card with extFAT

(To make a USB thumb drive or other disk to use with Windows, you may have to pick Master Boot Record as the scheme and ExFAT for cross-platform compatibility.)

now it is readable for cross platforms

http://www.macworld.com/article/2987201/operating-systems/hands-on-with-disk-uti lity-in-el-capitan-tool-for-storage-devi…

This will work temporarily: http://www.paragon-software.com/home/ntfs-mac/download.html

Hopefully they will fix this.

Mmmm. I have just tried to backup my files on Seagate which I have always been able to until E l Capitan. NOW - I can't do it. I have the time machine but I need this to work. Please help.

SSame problem and the paragon link making the Mac work with ntfssystem worked perfectly back in business. Just download this install, follow restart instructions and your read only will return to read and write. Thank god!!! http://www.seagate.com/gb/en/support/downloads/item/ntfs-driver-for-mac-os-maste r-dl/

good luck, it worked perfectly at solving exact same issue you have

Please try these options http://kecodoc.com/free-ntfs-read-and-write-solutions-in-mac-os-x-el-capitan-or- older/