What is up with these Sandish flash drives!! They quit accepting files.

[macx]

Have 2 Sandisk flash drives.

Worked great for awhile.

One quit before I found out I should use a USB removal tool for taking them off of an XP system.

So - got another, religiously use the removal tool.
Also used it on a 98SE system where everyone says
no tool required.

Now the 2nd one has quit accepting files.
I can still read and copy FROM the drive,
but can't copy or move anything TO the drive.
And it's only 1/2 full.

Says the file can't be created.

What's up with these things!?

Should I get another brand, or can
I fix this one? And if so, how do
I avoid this?

Thanks!