With the Berhinger UFX1204 (USB 2.0) I can record @16 bits on most flash drives but I'd really like to record @24 bits. The UFX us supposed to record @24 bits if the flash drive is fast enough. My fastest drive is a Lexar 32GB S45 Jumpdrive USB 3.0 that reads @150MB/s & writes @30MB/s. I would think that it would be fast enough but Behringer thinks not. I won't be happy if I have to use an external drive.

HDs connected via USB-2 top out around ~40MB/Sec.
Have you benchmarked the flash-drive when connected via USB-2?
To achieve 150MB/Sec on reads, it has to be connected via USB-3.
 
If you want to max out speed and keep a somewhat small foot-print:
Put a SSD into a USB enclosure.  (120GB SSD and nice external enclosure can be had for under $100)
You won't be getting near the full speed of the SSD, but it'll deliver all the USB-2 port can handle.
If the mixer isn't able to record at 24Bits with the SSD, it'll never effectively be able to do so...

Flash drives = oxymoron.   The bigger they are the slower they are. I'm assuming you are not using a desktop.

Sandisk Extreme 
Sandisk Extreme
Lexar JumpDrive
I've assumed write speed is more critical than read speed since this is for recording.

Realize that the speeds quoted are when connected to a USB-3 controller.  
No USB-2 controller is going to sustain 150MB/Sec (under any circumstance).
USB-2 tops out at about 40MB/Sec.
 
You can benchmark the flash-drive's write speed when connected to a USB-2 port.

Thanks guys!
 
@Jim= Your point about USB 2.0 is what I'm worried about. Behringer is so vague on specs & details I suspect the UFX isn't capable of recording @24bits on flash drives. I may try the SSD route tho.
 
@fret_man=Those are the ones I was looking at trying but my local shop wants full price (2x over mail order).
 
@kitekrazy1=Actually the bigger the flash drive the faster the speed... according to the packaging.

Then there are the USB 3.0 SSD drives like the VisionTek, that read up to 450 MB/sec and write up to 155 MB/sec:
 
https://www.visiontek.com/solid-state-drives/usb-3-0-ssds.html