Independent of the format of the drive, you would be best to perform a check of the file system on it.

Assuming that you have SSH enabled, you should be able to connect in and use a series of commands to unmount, check the file system (if it is a system that is supported for checking on the pogoplug of choice), and then mounting as r/w.

Note, if this is an HFS+ w/ journaling, there are some known odd behaviors, and it would be best to eject & attach to a Mac.