We upgraded my OS to Seirra and ever since then the Pegasus has become Read Only - without the ability to change to read and write.
Per Apple's instructions we reformatted the Pegasus ensured it was formatted to Mac OS Extened (Journaled) it worked for a day after transferrring files back, after restarting my OS the Pegasus went back to Mac OS Extended - without the (Journaled) and is again "read only".
Per Apple this issue lies with Promises - and we need to get an updated utility or firmware in order to ensure that we can run Seirra! Please provide the update! Thank you.