Your diagnostics show that flash drive really appears to have dropped offline:
Oct 8 03:30:55 AKserver kernel: usb 1-5: device not accepting address 2, error -71
Oct 8 03:30:55 AKserver kernel: usb 1-5: USB disconnect, device number 2
Oct 8 03:30:55 AKserver kernel: device offline error, dev sda, sector 557311 op 0x1:(WRITE) flags 0x800 phys_seg 1 prio class 0
Oct 8 03:30:55 AKserver kernel: write_file: /w..9/...
Oct 8 03:30:55 AKserver kernel: md: could not write superblock file: //..t/...
Oct 8 03:30:55 AKserver kernel: device offline error, dev sda, sector 375650 op 0x1:(WRITE) flags 0x100000 phys_seg 1 prio class 0
Oct 8 03:30:55 AKserver kernel: Buffer I/O error on dev sda1, logical block 375587, lost async page write
Oct 8 03:30:55 AKserver emhttpd: Unregistered - missing key file (ENOKEYFILE2)
Oct 8 03:30:55 AKserver kernel: FAT-fs (sda1): Directory bread(block 375584) failed
Oct 8 03:30:55 AKserver kernel: FAT-fs (sda1): Directory bread(block 375585) failed
Oc
You should probably plug it into a PC/Mac and check it is readable.