When you add your third party storage drivers to a windows install (F6 if you catch it) using a USB floppy sometimes the install will fail after the format stage. At this point it is unable to communicate with the USB floppy even though it initially could. You can help it by adding the following to your ‘textsetup.oem’:

id = USB\VID_03F0&PID_2001, usbstor #--HPid = USB\VID_054C&PID_002C, usbstor #--Sonyid = USB\VID_057B&PID_0001, usbstor #--Y-E Dataid = USB\VID_0409&PID_0040, usbstor #--NECid = USB\VID_0424&PID_0FDC, usbstor #--SMSCid = USB\VID_08BD&PID_1100, usbstor #--Iomegaid = USB\VID_055D&PID_2020, usbstor #--Samsung

This will give windows a nudge in the right direction. In my case I only needed the Sony line for my Dell external floppy drive…