Hard disk where your library is located is unavailable. It may be ....

Error message that I am receiving:
The hard disk where your library is located may be full or unavailable or permissions may have changed. To avoid losing your work, quit Final Cut Pro.

Our power went off briefly yesterday and this problem appeared shortly after the power failure. However, The library opens the project and I can skim through it but when I try to copy or do anything to the project this message appears.

What I have tried:
1. Checked the external hard drive where the project is located. No problems there. 4 TB of space available, permissions are correctly set, no issue copying and moving files to and from the disk.

2. Re-booted the Power Mac Pro (Mid 2010) 2 x 2.4 GHz Quad-Core Intel Xeon with 8 GB Memory and AMD Radeon HD 7950 3072MB Graphics card - Which has been running FCPX for years.

3. Zapped the PRAM

4. Ran Digital Rebellion to check for corrupt files. Found none.

5. Copied the entire Library to an internal hard drive. Turned off the external hard drive. Re-opened FCPX running the library off of the internal drive. Had the same error message.

6. Upgraded MacOS Sierra 10.12 from Mountain Lion.

None of the above has worked.

Thought I might try uninstalling and re-installing FCPX but not sure that I can find my serial number. I've had and paid for every upgrade since FCP 1.25 but not sure where my current serial number is.

Anyone else having this issue? Anyone have any solutions or suggestions?

You don't have to worry about the serial number for FCPX I don't even think there is one it's all dependent on your Apple ID.
If you delete it and go to the App store (for Mac) you should see install and not $299.00 I just did this yesterday with one of my customers.
After that if it still gives you errors try and open from a back up you might lose maybe 15 minutes of work.
The Back ups SHOULD be in your movies folder/Final Cut Backups

------------------------
Dean

"When I see you floating down the gutter I'll give you a bottle of wine."
Captain Beefheart, Trout Mask Replica.