LSI SAS 3008 Unable to find a properly formatted USB Disk

I've noticed multiple error messages, related to LSI_SAS3i in "Administrative Events" view (whole story here). 

• Windows 10 x64 version 1703, build 15063.966
• My T7910 got LSI SAS 3008 controller, with 6 HDDs attached. Two of them are in RAID0.
• System works OK, and I'm able to read/write to all my HDDs.
• SMART looks OK for all drives.

So, I've decided to check RAID controller log. I was unable to find any tool, that allows to do that from OS, but I found "Save Controller Events" item in pre-boot LSI configuration tool:Save controller events

However, all I get is "Unable to find a properly formatted USB Disk" error message:
Unable to find a properly formatted USB disk

The USB drive I've used is FAT32, 8GB.

My question is: what is "properly formatted"? FAT16? Smaller disk size? Specific cluster size?

After a bit of googling, I've found several non-dell manuals, describing similar menu: 1 (page 44), 2 (search for "Save Controller Events" on that page). However, they don't describe anything like this error message. And they don't say anything about "proper formatting".