Excel 2011 Opens Spreadsheet As Read-Only

Excel 2011 Opens Spreadsheet As Read-Only

I have a spreadsheet (.xlsx) file that, when I open it on my MBP running
Excel 2011 14.3.6, I get a pop-up telling me the file is locked and can
only be opened in read-only mode. About a minute later, Excel opens
another pop-up and this time says that the file is unlocked and asks if I
want to edit it.
I've triple-checked permissions on the file and it's definitely editable
by me. I've also verified in Get Info -> General that it's not locked.
The file is located on a USB flash drive. I created a copy of this file on
the local drive (/Users/aj) and it will open fine without the warning.
Other Excel files saved on the USB flash drive open without this warning.
A copy of this file on the USB drive still has the warning.
What can I do to remove the warning? I use this file all the time and it's
super-annoying.