I use a WD 300 gb usb disk for backup, storage etc. No problems until a couple of weeks ago. I can access it with file managers, gparted, fdisk etc with no problems. But now I am unable to write more than the occasional file to the disk. There seems to be no consistency as to what will or won't write. I get a message that the operation is not permitted. This occurs under either user or root.

It happens in both 13 and 13.1. No other issues have come up. The internal hard disk in the laptop remains normal. All other usb periph's are working properly.

Any bright ideas will be greatly appreciated.

Maybe the disk is wonky. Try it on another computer.

Disk runs fine with another system (Windows, only have the one with Slack on it). Perhaps a re-format would help? If so, what should I format to (i.e. what file system)?

What is it now? If you need to use it on Windows you can use ntfs or fat. Both should be stable on Linux. If it will be a Linux only disk, then try ext2/3/4.

If it runs fine on another system then the problem may be caused by the first computer rather than by the device itself -- in which case reformatting would gain nothing except to muddy the problem investigation waters.

How about testing another USB HDD in the first computer or re-testing the original USB HDD in the first computer but plugged into a different port or with a different (ideally "known good") cable.

USB HDDs can be on the limit of USB port's power supply capacity so, if it has the option of an external power supply or a second USB cable for extra power supply, try using those in the re-tests.