The PST may need a lot of space during use or compacting. Keep it on the HDD so the drive letter won't change and there is an abundance of space for Outlook to compact and more.
As to read only, there are tomes on the web about removing readonly flags.
Finally, I recall no support for this on FAT drives.
I've decided to use MS Outlook as my primary mail program on my Win 8.1 64-bit Pro desktop and Laptop. Previously I used Gmail on the laptop and Outlook 2007 on my desktop. Outlook 2010 is used on the laptop. I've used Outlook .pst files as my primary mail archives on all of my Windows desktops.
After installing Outlook 2010 and successfully on my laptop accessing my new mail on my ISP's mail server (POP3), I moved the current archive on my desktop to a USB flash drive using Windows Explorer. The archive was then moved to my laptop from the USB flash drive, also with Windows Explorer. Attempts to use the archive on my laptop failed with "an access denied" message. I researched this issue online, but none of the remedies worked. After more failed attempts to access .pst files copied from a USB flash drive, I inspected the copied file and discovered that the attributes had changed. Specifically, write access was removed from the .pst on flash drive. I'm stumped and hope that someone can help me. Thanks for your support.